Update from me:
Unicredit HVB (HYVEDEMM455) <-> bunq does seem to work reliably with SEPA INST again since at least 11.Feb
I have no issues anymore.
I still have no information from either HVB nor bunq despite inquiring both sides every week.
I‘ll stop that now since it seems to work again for good and I recon I will not getting any in detail respsonse ever. (Which is sad).