Yeah, this has been on going for a while now. I’ve had to use another bank (still on MasterCard) to make any purchases through Google Play so definitely seems like a Monzo issue.
Start a Chat through help, hopefully will get a move on the more people suffer from it.
Google Wallet supports card details and cards enabled for contactless.
You should be able to disable contactless whilst keeping it the card used for online play store purchases and subscriptions.
Once you have disabled contactless in Wallet, try going back to Monzo app and tap “add to Google Pay/Wallet” button to add it back to wallet as contactless enabled payment card.