x86 bootloaders make a lot of assumptions about the state the system is in when the BIOS passes control over to it.. legacy structures in memory, register contents, and how other legacy hardware is configured.
I guess when this system goes to the F12 menu it takes some extra precautions to ensure the legacy environment is more compatible.. it's really hard to say why.
A lot of newer x86 firmware is actually UEFI/EFI with legacy BIOS emulation as a module.
It's definitely frustrating, and it may indeed be a bug in OpenBSD's bootloader, however it does boot on a large amount of systems.. the problem is likely pretty obscure and will be difficult to hunt down, contacting a developer who has recently committed in that area may be able to help isolate it.
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