Multiple notifications arrive all at once, from multiple apps
Going by your description of not happening when you " move or shake the phone" , it appears to be related to Doze function of Android, which may be the cause

You would need to exclude these apps from the Doze feature. I am not sure how to do that on Nougat but on Marshmallow,   Is there a way to exclude an app from Doze?  should help you exclude those apps ( steps to exclude would probably be similar on Nougat )

Edit: Doze behaviour has been changed in Nougat. Doze is implemented in two stages


  When a device is on battery power, and the screen has been off for a certain time, the device enters Doze and applies the first subset of restrictions: It shuts off app network access, and defers jobs and syncs. If the device is stationary for a certain time after entering Doze, the system applies the rest of the Doze restrictions to PowerManager.WakeLock, AlarmManager alarms, GPS, and Wi-Fi scans. 


My understanding is that this change does not help your problem , without excluding from Doze