All it takes is for to have been incorrectly installed once (too tight or too loose). The crank material is much softer than the BB spindle so will deform away from the ideal shape very easily.
Once that happens it's bin time.
Oh great... :(
How can I test for deformation? Just hold it next to the drive side I guess?
Can you get a good photo of the square taper? Or yes, compare it to the taper of the other crank.
If it has come off more than once I wouldn't hold much hope.
@JWestland started
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All it takes is for to have been incorrectly installed once (too tight or too loose). The crank material is much softer than the BB spindle so will deform away from the ideal shape very easily.
Once that happens it's bin time.