I honestly hope this isn't related to your problem, but I did make a change to the amd64 cdrom building recently that is in this area and may be the issue.
https://mail-index.netbsd.org/source-changes/2021/02/06/msg126676.html I thought I was only affecting the boot-com.iso install cd, but this change should probably be examined to make sure it isn't causing your problem. Darrin > Has something changed recently in the land of serial consoles? > > I am pretty sure that once I enabled serial console redirection > "after boot" in the "bios" of this amd64 uefi booting server, with > a serial port plugged in, I would have a serial console with the > default /boot.cfg. > > After getting access to the building(!), it seems I now need to add > consdev com0,115200 to each menu item in boot.cfg. (Putting it > on a line on its own seems insufficient - is that a bug?) > > > Cheers, > > Patrick > >