So, I did find a forum thread with others who have that same issue starting back from april 17 2025, confirming that DNSviz report. There, solutions are provided with a change in unbound.conf: 'harden-algo-downgrade: no' that would effectively mean something like 'if alg 13 is okay then ignore alg 8'. And then the thread continious with lots more changes that I don't understand 
I'll report this to my provider and link the forum post that shows it is actually an issue with them, with the zone configuration, and see if that resolves anything.
Edit: I found 1 other report of this at my provider from 29 days ago but no reply there, unfortunately. Sent a report myself as well. The Dutch forum thread that also provides a soluition: https://gathering.tweakers.net/forum/list_messages/1844277?data[filter_keywords]=odido
I think this topic can be closed.