Ah, stuck in what i call neutral. Chain width VS ring c-c dimension can come into play here. Chain angle as you shift too, you did say it's only when you're in the outsides of the cassette. I don't think that anythings going on that you don't already kind of know about. You're right in that i initially read into your OP what I wanted, sorry.

But as I've said before in other posts, my replies are also for those reading. While your problem is real to you it is not a frequent one for most. Inability to get the chain to derail off what ever ring it's on because of high pedaling pressure (often because the rider waited too long and has a low cadence to begin with)is a very common issue. one that "simple" skill improvements can go a long ways to fix. Andy.