It seems like TfL payments are accepted even though the card doesn’t have any funds on it - my Monzo was near empty when I used it on the subway, and surprisingly the transactions came through and my balance is now -4,99.
Is this the expected behavior or a bug?
It is expected behaviour due to the particular ways TfL process payments and the distinct status of public transit system payments in the card networks.
Reconciliation of TfL payments occur at night, and settlement of that payments happens well after. In this case, it’s to be expected, and a complete pain in the rear, because they don’t charge when you tap out… they charge for a whole day’s worth of travelling.
There are industries that have exceptions to some of the main card scheme rules, such as low value transport (e.g. TfL) and purchases on board a plane, both of which are allowed to be authorised offline, even if Monzo wishes their card transactions to be carried out online.
Whilst the one-off payment at the end of the day can be a pain, it’s very useful for people using a foreign card who might otherwise be liable for transaction fees on each individual transaction.