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The FD was adjusted correctly
That's what they all say :P
Cage wasn't bent or was something flexing that would allow the chain drop even though it appeared limited correctly in a workstand? Seems an odd one to happen in both directions. What sort of frame? Not old steel or cheap carbon? You weren't running some super RD cage that had really slack springs like a CeramicSpeed hanger?
hippy-
Nope, Cervelo and Brommers Bikes double oversized tubing steel frame, standard RDs in both cases. No further issues after I fitted Rotor chainrings, and the Praxis rings have worked fine with a mechanical DA9000 groupset (except for the crankset - SISL2 crank arms with a P2M spider) after I'd removed them from one of the Di2 bikes. Makes absolutely no sense, I know, but there it is.
Brommers
Chain dropping off the rings at both extremes. The FD was adjusted correctly.