I remember having a banding issue like this a long time ago at my old address, if I recall I was stuck on 60/20 instead of getting 80/20 which I could've achieved. It was caused by a faulty faceplate filter which I replaced, the one the engineer installed was faulty which caused DLM to intervene. I got no where with them however, then they called a few months later to ask why I rated them bad in the survey after the phone call haha (BT Business at the time).
Fortunately after what must've been a year, DLM apparently reset and I was on a wild open profile, then shortly after that on the normal 80/20 profile. Not sure to this day what caused that.