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boredom-
Remember to check both sides, it could equally be the NDS+axle moving relative to the DS as the NDS moving relative to the DS+axle.
For anybody not familiar with the history, Shimano quickly but quietly increased the spline length of Octalink after this kind of failure started happening, but the fix was applied only to the road and MTB kit, not the track cranks. Presumably investing in new forge tools for the tiny track market was too much for them, and when used as intended on a velodrome the track cranks are not generally subjected to the shock loads which are most likely to precipitate failure.
gbj_tester
If it's moving relative to the drive side, it's most likely fucked internal spline in the crank itself, usually caused by the bolt having worked loose allowing the crank to move on the axle. Take it apart and inspect, although it's likely already too far gone to save.