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@moth
Where the bb taper meets the crank it tends to slightly bite into the metal of the crank (not sure if this is plastic deformation or fretting). With an ISO crank on a JIS bb this can leave an un-eaten ridge on the furthest out edge of the crank's taper faces. Fit the same crank to an ISO bb and the ridge will meet the bb taper before the rest of the crank's faces. This will give a fit that works loose quickly and/or has extra high stress on that part of the cranks promoting fatigue cracks.
Filing off the ridge can work or can destroy the cranks. (I've gotten away with it on one pair of very cheap cranks, and years ago ruined another pair.)
First step is to look carefully at the cranks.