Upstream noise margin just jumped

[#1]

I'm hoping someone from PN can check something for me please:
After over 20 days of stable connection my router resynced yesterday but my upstream noise margin has jumped from about 5dB to 12dB (upstream sync rate going from about 950 to 500). This seems like a remarkable jump so I just wanted to check that I hadn't been randomly banded (or anything else obvious from PN's end) before I go trying to find other causes.
Many thanks.

[#2]

Ok, having talked to the helpline people, they have told me that my upstream is currently unbanded. I'd still like this confirmed here by a PN bod if possible please.
So assuming that's the case I now need to figure out why my upstream noise margin jumped from 5dB to 12dB. A clue is that after 22 days of stable connection my router resynced 3 times in quick succession and is now back to over 20 hours of stable connection.
If I truly am unbanded then theoretically my upstream noise margin should naturally drop back towards 5dB assuming the connection remains stable.
Very odd.