I believe Revolut is working as intended.

The root issue is that BOI have/had an issue trying to retrieve balances. At which point they have two options: prevent transactions if the balance cannot be verified (which would deny people access to their own money), or allow transactions if the balance cannot be verified (which enables people to go overdrawn).

SEPA transfers are near-instantaneous, this isn't access to funds that haven't cleared, it's access to funds which have already been transferred. None of this is really specific to Revolut, they're just the most common reason for "the man on the street" to have a second account to receive a transfer.