User "jomofo" concludes that it's a Logic Bug but has not been successful logging a ticket with Apple. This seems very odd to me as it seems like such a basic operation.
But also maybe I added in too many automation points, but again, that seem basic for the program to handle esp this early in the session.
I don't want to give in and delete the automation just yet but it seems there is no solution.
From user jomofo:
"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"