Over what distance? Sounds like something is broken… Sure all the pairs are punched down in the right order, cables aren’t crushed, etc.?

I’ve had 2.5GbE running over old in-wall Cat5 in old buildings on short to medium (15-30m) runs with no issues…

Roughly 15 metres. Almost certainly a cabling issue, although frustratingly the electrician who installed it says that they think it's fine. I've got a ubiquiti U7 AP connected to a USW lite 8 via POE, and it negotiates GbE, but then falls back to E.