Assuming that the chainline is within the normal range, i.e. that the bb spindle length is correct, it sounds like possibly just a bent tooth on the big ring.
That's all it takes to derail the chain. I deliberately bend teeth on the middle ring when downshifting needs to be more assertive.
Even if the tip of a tooth is bent over, that can significantly assist the downshifting process to a fault, especially as the big ring causes maximal cross-chaining when using the bigger cogs in back.
The fact that it's BioPace should have no effect, since while the gap between the teeth and the front derailer cage varies, there is normally no contact between the chain and the cage except while shifting.

Look for a suspect tooth on the big ring, and bend or bevel the right-hand side of any distorted tooth, or you could slightly bevel all of them to assist chain retention.
Out on the road, I've used a rock against the rotating chainring to bevel all the teeth at once, but with this side of the teeth the crankarm would get in the way unless the ring is removed for some tooth massaging.
Any missing or broken-off teeth will also allow the chain to cross over when the chain is crossed.
Start with a clean chainring and use magnification and good light as needed to discern any bad teeth.