Alright, trying one more thing. Let me know if it works now (if so, great, if not, I might need to borrow that Mac again and figure out what went wrong)
Damn. I hoped re-exporting the original that worked and adding the missing _CodeSignature folder from the Mac export would be a workaround, but if it needs a new signature each time then that's not an option. And I don't have access to the borrowed Mac so it could be a while before I can find a proper fix.