Just changed over my Mōjāv partition d0s2 to APFS that I have done a million times and it will not boot. And doesn't show on the Desktop, but does in the Picker/System Manager at Startup and in the SysPrefs, but won't boot.There is no reason to cling to HFS+ with Mojave. It simply makes updates a PITA.
I have had nothing but problems since the Apple 2020-005/6 & Safari debacle. It's as if the issues glued themselves to the Motherboard. The bit above was trying to start from a clean slate . . . but. Reverted to an HFS+ and it boots just fine. Like I said I have done this many times with no issues. I wonder if Apple is trying to thwart Hackintosh's?!