Yeah, that’s definitely bad news about the CP/M cartridge. Sounds like you really gave it a thorough test across a whole lineup of machines, too. At that point it does seem pretty likely the cartridge itself is the culprit.
On the bright side, it really sounds like you’ve got an impressive retro collection there—five breadbins, a C64C, a C64U… that’s seriously cool.
Trying it on a Commodore 128 makes a lot of sense, and honestly the Osborne 1 would be a great option as well. That one especially feels like a natural fit for CP/M, so it’d be fun to see it running there if you can get it set up.