Nothing will happen automatically.

A button to "open mediation" will appear though, which the buyer can press, so a dispute is started after which most likely the mediator will suggest that the btc seller pays a penalty to the buyer, for late release.

There is an edge case, where the buyer sent for example a sepa payment very close to the trade period end, at which point the seller is entitled to at least a few days of delay to get outside of the chargeback window.

In that case the seller cannot be penalized for a "late release", as the liability is with the buyer not having sent fiat asap.

I would argue that a buyer sending a sepa payment very close to the trading period end, that arrives past that limit, or just barely within in, could be penalized for late payment.