Interesting, it was happening repeatedly to me, and after several startup and shutdown cycles since I had done a few flights without going back to the menu.
Notes I should have included in my first post: using the Cargomaster, MSFS 2024 SU4 release, and yes the tablet to manually trigger the failure. I had thought it would be in an area using common code across variants, but I did not actually test that.
Is there anything I can do to help troubleshoot it? The first time I triggered the failure was actually the longest it took to happen; I had a few seconds to play with the throttle and see there was no engine response, and right around when I moved the emergency lever was when it died. I had thought I had done something wrong until I noticed both fuel tanks were at zero.
Subsequent failures of the FCU produced quicker, but not quite immediate results. Sometimes it would instantly zero, on at least one occasion I saw the tanks empty in a few discrete steps.