Automatically Mount Encrypted Core Storage Volume (10.14.4+)
It appears to be a bug that was introduced in macOS 10.14.4. In this discussion some users report that the problem went away by itself, a recent poster found the problem was solved in the latest 10.14.5 beta, and others have resorted to using the third-party app AutoMounter.

In my case, I have a SD drive permanently inserted into the SD card reader. That worked perfectly for years. Since the update it would auto-mount only about 50% of all reboots.

Therefore, it seems your best bet will be to wait for macOS 10.14.5 and report the issue to Apple. Another thing that you could try is to delete and re-enter the passphrase in your Keychain Access.app. Concerning your comment, the necessary steps would be: 


Open Keychain Access.app
Find the key by entering the volume's name in the search field
Select &amp; delete the key
Open Disk Utility.app
Mount the volume in question and check the "Save passphrase" box
Go back to Keychain Access.app and locate the volume key again
Drag &amp; drop they volume key into the "System" category in the left sidebar of Keychain Access.app. This step is necessary to enable auto-mounting.


P.S.: I experience another issue, which I suspect is related to this one. Possibly a new UUID is assigned to the drive which could be the reason it is not auto-mounted anymore.