I am experiencing the same problem in France. I cannot move my insurance standing order, (Allianz), my mobile phone, (Free), or my internet connection, (FreeBox), although the latter will except N26! However, other service providers accepted Bunq no problem, including our electricity supplier, (EDF), which I had thought would be the most difficult.
As far as I am aware, the refusal to accept a valid EU IBAN is a violation of current legislation as has been previously pointed out. I have written to both Free and Allianz asking them to explain their position and have had zero success so far, the most recent message from Allianz being that "we are looking at it and will contact you again shortly". That was a month ago.
Instead of trying to fight Free, (mobile), I tried to move my mobile phone contract, also with no success. Connected to the customer support centre of Red, (SFR), for example, when I asked them to explain why their web site would not accept a non French IBAN they simply hung up on me. Nice.