I've been thinking. Now that you have access to the sector contents (via USB), you could dd(1) the leading sectors to a file for analysis. It is possible that you may find an indicator of why an otherwise valid MBR will boot from USB but not from ATA.
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Edited to add:
It's a long shot. It could very well be that BIOS emulation is broken on these machines, and they require EFI boot from ATA connected hardware.
EFI boot is possible, with -current, but it requires some jumping through hoops.
This "howto" may not be up to date, as EFI boot is very much a moving target.
https://blog.jasper.la/openbsd-uefi-bootloader-howto/