Macbook Pro starts by itself, with lid closed, since upgrading to Mojave
I ran into the exact same issue, clamshell closed and in my bag (power adapter plugged into the laptop with top of bag unzipped) - and realized that "Enable Power Nap while plugged into a power adapter" was somehow set to on in the Energy Saver preference pane.

I also had:

2019-06-19 21:32:48 -0400 Assertions            PID 58(powerd) Created InternalPreventSleep "com.apple.powermanagement.darkwakelinger" 00:00:00  id:0x0xd0000927f [System: PrevIdle DeclUser SRPrevSleep kCPU kDisp]          
2019-06-19 21:32:48 -0400 Assertions            Summary- [System: PrevIdle DeclUser SRPrevSleep kCPU kDisp] Using Batt(Charge: 100)          
2019-06-19 21:32:57 -0400 Assertions            PID 3171(AddressBookSourceSync) Released PreventUserIdleSystemSleep "Address Book Source Sync" 00:00:30  id:0x0x100009276 [System: DeclUser SRPrevSleep kCPU kDisp]          
2019-06-19 21:32:57 -0400 Assertions            PID 270(UserEventAgent) Released BackgroundTask "com.apple.AddressBook.ScheduledSync" 00:00:30  id:0x0xb00009277 [System: DeclUser SRPrevSleep kCPU kDisp]          
2019-06-19 21:33:03 -0400 Assertions            PID 58(powerd) TimedOut InternalPreventSleep "com.apple.powermanagement.darkwakelinger" 00:00:15  id:0x0xd0000927f [System: DeclUser SRPrevSleep kCPU kDisp]          
2019-06-19 21:33:03 -0400 Assertions            Summary- [System: DeclUser kDisp] Using Batt(Charge: 100)          
Sleep/Wakes since boot at 2019-06-18 22:17:25 -0400 :32   Dark Wake Count in this sleep cycle:27

Time stamp                Domain                Message                                                                     Duration    Delay     
==========                ======                =======                                                                     ========    =====     
2019-06-19 21:33:03 -0400 Sleep                 Entering Sleep state due to 'Clamshell Sleep':TCPKeepAlive=active Using Batt (Charge:100%) 22 secs   
2019-06-19 21:33:08 -0400 Wake Requests         [*proc=mDNSResponder request=Maintenance inDelta=2290] [proc=powerd request=TCPKATurnOff inDelta=43255]           
2019-06-19 21:33:08 -0400 PM Client Acks        Delays to Sleep notifications: [com.apple.apsd is slow(3123 ms)] [mDNSResponder is slow(5014 ms)]           
2019-06-19 21:33:24 -0400 Kernel Client Acks    Delays to Sleep notifications: [powerd is slow(5018 ms)] [UPSB driver is slow(msg: SetState to 2)(935 ms)] [UPSB driver is slow(msg: SetState to 2)(934 ms)] [AppleHPMIECS driver is slow(msg: WillChangeState to 2)(1088 ms)] [AppleHPMIECS driver is slow(msg: WillChangeState to 2)(1088 ms)] [AppleUSBVHCIBCE driver is slow(msg: SetState to 1)(310 ms)] [RP01 driver is slow(msg: SetState to 0)(1007 ms)]           
2019-06-19 21:33:24 -0400 Assertions            Kernel Idle sleep preventers: IODisplayWrangler          
2019-06-19 21:33:24 -0400 Notification          Display is turned on                                                                  
2019-06-19 21:33:24 -0400 Assertions            PID 183(mDNSResponder) Created MaintenanceWake "mDNSResponder:maintenance" 00:00:00  id:0x0xd00009297 [System: DeclUser kDisp]          
2019-06-19 21:33:24 -0400 Assertions            PID 183(mDNSResponder) Released MaintenanceWake "mDNSResponder:maintenance" 00:00:00  id:0x0xd00009297 [System: DeclUser kDisp]          
2019-06-19 21:33:25 -0400 Wake                  Wake from Normal Sleep [CDNVA] due to EC.LidOpen/Lid Open: Using BATT (Charge:100%) 37 secs   
2019-06-19 21:33:25 -0400 HibernateStats        hibmode=3 standbydelay=10800                                                          363           
2019-06-19 21:33:25 -0400 WakeTime              WakeTime: 2.788 sec                                                                   
2019-06-19 21:33:25 -0400 Kernel Client Acks    Delays to Wake notifications: [UPSB driver is slow(msg: SetState to 2)(544 ms)] [UPSB driver is slow(msg: SetState to 2)(543 ms)] [IGPU driver is slow(msg: SetState to 2)(558 ms)] [GFX0 driver is slow(msg: SetState to 2)(552 ms)] [HDAU driver is slow(msg: SetState to 2)(552 ms)] [ADIO driver is slow(msg: SetState to 2)(541 ms)] [AMDFramebufferVIB driver is slow(msg: SetState to 2)(765 ms)]           
2019-06-19 21:33:26 -0400 Assertions            PID 320(NotificationCenter) Created DisplayWake "com.apple.notificationcenter.sleeping-note" 00:00:00  id:0x0xe000092a9 [System: PrevIdle DeclUser DispWake IPushSrvc kCPU kDisp]          
2019-06-19 21:33:31 -0400 Assertions            PID 320(NotificationCenter) Released DisplayWake "com.apple.notificationcenter.sleeping-note" 00:00:05  id:0x0xe000092a9 [System: DeclUser kDisp]          
Sleep/Wakes since boot at 2019-06-18 22:17:25 -0400 :33   Dark Wake Count in this sleep cycle:27

Time stamp                Domain                Message                                                                     Duration    Delay     
==========                ======                =======                                                                     ========    =====     
2019-06-19 21:33:47 -0400 Notification          Display is turned off                                                                 
2019-06-19 21:33:47 -0400 Assertions            PID 101(hidd) Summary UserIsActive "com.apple.iohideventsystem.queue.tickle.4294984775.11" 00:00:19  id:0x0x900008547 [System: DeclUser kDisp]          
2019-06-19 21:33:47 -0400 Assertions            Kernel Idle sleep preventers: -None-          
2019-06-19 21:33:47 -0400 Assertions            PID 58(powerd) Created InternalPreventSleep "com.apple.powermanagement.darkwakelinger" 00:00:00  id:0x0xd000092b8 [System: PrevIdle DeclUser SRPrevSleep kCPU kDisp]          
2019-06-19 21:34:02 -0400 Assertions            PID 58(powerd) TimedOut InternalPreventSleep "com.apple.powermanagement.darkwakelinger" 00:00:15  id:0x0xd000092b8 [System: DeclUser SRPrevSleep kCPU kDisp]          
2019-06-19 21:34:02 -0400 Sleep                 Entering Sleep state due to 'Clamshell Sleep':TCPKeepAlive=active Using Batt (Charge:100%) 420 secs  
2019-06-19 21:34:07 -0400 Wake Requests         [*proc=mDNSResponder request=Maintenance inDelta=2237] [proc=powerd request=TCPKATurnOff inDelta=43255]           
2019-06-19 21:34:07 -0400 PM Client Acks        Delays to Sleep notifications: [com.apple.apsd is slow(3131 ms)] [mDNSResponder is slow(5015 ms)]           


after running pmset -g log

This was from maybe 10 minutes before I opened the laptop, and it had been spitting out similar entries for the prior ~24 hours worth of "sleep" I thought the machine was getting.