Thanks Hugh, yes that did fix it. I adjusted the reference size to .795 and it works. It got me wondering what I was doing differently in this project compared to others in the past as I’ve never had this occur before. Eventually, I realised (for reasons I won’t bore you with) that by running the timeline size at 4K Full (4096×2160) and an output scaling resolution customised to 4K Scope (4096×1716) it was causing the issue. By changing the timeline resolution to 4K Scope the problem also goes away.
Stills are spot on either way.