Here is a situation:
iMac 5K from 2014 with Fusion drive, fusion drive is formatted as APFS, volume is called DATA and there is nothing on it.
Firmware password is set.
External USB drive (SSD) is connected, formatted as APFS and has Mojave on it. Drive is also encrypted with Filevault.
SSD is selected in System Preferences as the startup disk.
When I turn on the computer, it works fine, boots from USB drive.
When I then press Restart... from Mojave, the iMac restarts and shows me a blinking folder icon with a question mark.
I can press the power button for 3 seconds to turn the iMac off, and when I turn it back on it boots fine.
Can I do anything to make it reboot as well?