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Try another wheel? Might be some weirdness with the free hub / cassette combo you are using?
I'm wondering this now. I've got digital calipers and just measured up the chain line. It does seem to be 43.5mm. Its hard to measure into the mid point between chain rings but it does look to be correct.
Wheels are second hand, its maybe possible the axle was re-assembled incorrectly at some point. This could put the cassette out maybe.
Its actually harder for it to slip in the big ring actually, it doesnt slip so much but i can hear more noise. This makes me suspect its the drive side cranks being to far inboard because the chain line looks more normal while in big ring but still not right.
when in the small ring it slips constantly in 11t and 12t