All the conditions for close the pocket were met as far as I'm able to tell, replay attached, can anyone tell me what went wrong?
Here is your answer. The brit built a forward observation post.
Many people dont know this but forward HQs where
ambient (civilian) houses get converted,
keep your territory connected until the FHQ gets destroyed, no matter if you are no longer connected to your main base territory.
There are 3 FHQs in CoH2 (soon):
1. Soviet FHQ from Urban Defense doc
2. Brit Observation Post from Tactical support doc
3. Wehr Resupply Station soon from Joint Operations & Festung Support doc
So not a bug, just a mechanic in the game
https://cohdb.com/replays/11854