I can still top-up my Monzo card using the latest android app (as of 19/10/2017).
Successfully topping up with a Lloyds VISA debit card which expired 07/17.
So far three successful top-ups since the card has expired.
Although the processor should check that a card isn’t expired, I would be more concerned with Lloyds still authorising transactions for the card!
I suggest you contact Lloyds to ask about this as well as the team looking into why you were allowed to enter an expired card at all 
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I don’t believe this to be a bug (at least on our side). It is likely that Lloyds told our top up processor about your new card and it’s all being handled transparently. This is a relatively unknown feature of a few issuers and merchants that allows some subscriptions and saved cards to be automatically updated when a card reaches its expiry without being cancelled. 
With some card issuers, you’ll even have the same long card number with just an updated expiry date and CVV.
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I’ve seen that done with continuous authority* but I didn’t think it could be done for one-off payments like that. Learn something new every day 
- It’s a right PITA too when that happens - I once had to cancel a credit card completely to stop a company taking money out of it.
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I think I have a few cards dotted about that are still ‘live’ as it were.