You already tried "restoring" the wallet by recreating the MultiSig using the original co-signer xpub + your hardware wallet didn't you?
Yes, I tried to recreate the multisig twice, it created a new wallet, the same new wallet twice
. I don't understand how the underlying seed would change... It has not, I have not factory-reset the Trezor in any way. I've only updated the firmware to latest...
I have a spare trezor-t in the box. I think I will try replicating my case... open it, create a multi-sig wallet, and then upgrade firmware to see if my issue can be replicated...