Quirky Performance/Playback issue

I have a C12.0.30 project with 40 tracks, with plugin processing on almost all of the tracks. This is on a brand new AMD 7700X with 32 Gb DDR5 RAM.

Plays fine (CPU overload light and wouldn’t play at all on my old i7) … but not from measure ? I get the CPU overload light, the play button turns green but the playhead doesn’t budge. The earliest it will start to play from is measure there is a CPU bump, but not huge, and the project plays from there, anywhere else in the project it runs without a CPU bump (steady as per pic) and without so much as a hic.

It’s not a big issue for me as I start most of my projects from measure 3 to accommodate lead-ins, but just wondering why this would be? (no issues with an empty or small project).