No, I didn’t miss it.
I suggested it for 2 reasons. Firstly every Openreach Engineer you’ve had put hasn’t been able to emulate the problems you say exists, which could point to the issue being on BT’s side.
Secondly your neighbour is only human and most likely has a pack mentality.
I give up. Clearly wasting my time posting here and reporting it to BT.
Clearly you didn't come here for advice. An ex Openreach FTTP engineer has suggested a test and you decide to ignore it. Fair enough. Your choice.