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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.
Try another wheel? Might be some weirdness with the free hub / cassette combo you are using?