The Monzo card might be somewhat “unusual” and unique compared to the rest of UK cards (though still within EMV specification), and as a result might be triggering some obscure bugs in the terminal’s software, that weren’t caught during development because the entire thing is held by duct tape and nobody working on it had any clue about proper software development practices.
This has happened before where the card not only crashed the terminal but also the computer connected to it, so I wouldn’t be surprised if a similar thing is happening here.