Boot stops at loader after build/install cycle

For some time (months, I think) a Pi4 running -current 
has been stopping at the loader prompt on first reboot
following a buildworld/buildkernel/installeverything
cycle. The system uses a USB mechanical disk, no microSD
and has never had trouble finding the disk.  

It seems to happen only on the first reboot, though 
I'm not absolutely certain  since I haven't been
keeping notes. 

Is this intentional behavior? 

Thanks for reading,

bob prohaska