I've seen where the bolt gets loose then after awhile the crank gets loose on the spindle, often damages the crank and it can't be effectively tightened again. But OP's situation is, it turns out, different. He doesn't have the standard square taper spindle.
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Quote de jyl
If only the drive side crank has play, I'll guess it is the crank bolt that is loose.
Even if the bolt is loose the crank should stay tight no?
Bad bearing in that pedal. Called it 
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