Wow, okay, so it only took me an hour or so to redo my tempo automation using the Tempo Operations (Edit > Tempo > Tempo Operations...) window. It's not quite as user friendly, but at least I don't have to fight with the tempo curve handles like I do in the global track.
I changed most of my curves to just 1/4 steps up and down over a bar. I have a couple of bigger drifts where I felt like it needed 1/8.
It actually worked! The clock doesn't drift out of sync now.
I speculate there's a bug in whatever they're doing to calculate the tempo from those continuous style curves in the global track that is additive in nature. The longer you playback the larger the error becomes. I don't have a guess as to why certain settings tweaks seem to amplify the error and make it more noticeable sooner in the playback.
And I guess bug doesn't exist if you use incremental steps up and down to simulate a curve rather than an actual curve.
Anyway, I've burned 3 late night sessions trying to figure this out, so while I'm perturbed, at least I have a workaround. On the bright side, it forced me to understand a ton of settings I never bothered to dig into before.