I think the post office gets paid fees by most banks for delivering cash services on an ad hoc basis like over the counter withdrawals etc, but they have no such agreement with Monzo so the hot coral physically doesn’t work for those cash services. A small minority of their cashiers are probably confused by that and think it just doesn’t work for anything at the post office.
I had an experience where I needed to withdraw a tenner and didn’t want to pay £2 to use the nearest ATM, so asked over the PO counter and the cashier said “no problem - just pop your card in and enter you pin”. I whipped out my Monzo card and he said “ohhh not that one, you can’t use that one to withdraw here”. I then got my nationwide card out and he said that one will work fine.
I’ve used the card to buy stuff there loads of times from the same guy, including paying for postage etc. It’s just cash services he said won’t physically work for Monzo cards at the PO.
I think the smartest thing you can do at these confused post offices when paying for stuff (not cash services) is to hide the card and slam it on the reader before they get a chance to think about whether it should be accepted or not