My vote is for a warped chainring(s) that cause the adjustment you've carefully made to depend on where in the rotation the chainwheel is.
if the chain is on the small chainwheel, and there's pressure from the FD, and the chainwheel warps to the right (outside), the chain could be pushed off the small chainwheel to the left.
It wouldn't take much of a warp if the FD is already touching the chain.
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Dale, NL4T