I'm wondering if you have some scapular locking issues. Sometimes on dips, the 'sagging into the bars' position itself locks your scaps back. On any kind of bench: barbell, dumbbell, close, wide... you have to consciously slam those scaps back and down.
Perhaps that's why sometimes lighter weight is ok too... you can stay locked. But once you get into higher reps or higher weight, your form loosens, and the scaps probably start moving around, leaving your shoulder open for the bad stuff.
Just a thought!
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