I was able to pay an expired lightning invoice in version 0.4.8, android.

1. Open an invoice in the app to reveal the payment screen.
2. Tap on the amount field to show the keyboard (I am not sure if the step is required, but it helps keep the app in focus - the screen must not go to sleep)
3. Wait until the invoice should have expired
4. The screen is still on the payment screen with the keyboard
5. Tap on the Pay button

Result: Payment goes through to my LND via BTCPayServer.

I would expect this to not go through because according to the bolt 11 spec:

A payer:
after the timestamp plus expiry has passed:
SHOULD NOT attempt a payment.

Source: https://github.com/lightningnetwork/lightning-rfc/blob/master/11-payment-encoding.md#payer--payee-requirements