Kinda off-topic, but actually I also once had a problem with it. I was never able to receive a re-fund for a train ticket from Deutsche Bahn that I had paid with Sofort, even after explaining and going over it with their "support" multiple times. Maybe I would've eventually be able to get it if I had been more persistent; I gave up after a while because it was a minuscule amount.
Granted, I use Sofort very often with bunq and that was the only problem I had, so it's not something that happens all the time, but maybe merchants are not completely ready for the solution that bunq implemented with Klarna, even though it is on paper a very good one.
What to make out of this? I'm not sure, but maybe a special page or official document from bunq that explains these things in very clear terms to merchants might already be a help. Even better if Klarna could also weigh in on this, because it is also their responsibility. They design how the system should work, and if they on the one hand cooperate with bunq to allow for such an implementation, but on the other hand don't force merchants to fully comply, that's clearly gonna lead to problems down the road. And one of the most prominent one's is to be found in this thread.