Eva Some merchants might do that. It's not particularly strange, the card networks like Mastercard just work like that and bunq is transparent and shows you and processes almost everything immediately.
The technical reason is that the first 50 Euros are just a kind of deposit that is then returned from the merchant to you, and after that the actual payment will get booked. With most types of merchants this shouldn't happen, but with some gas stations it might be the case.