Maybe we're overlooking the obvious, are you a full roller (initial track ring between the thumb and finger holes)?  --  JohnP

it has to do with either the drilling or your release.

but since you say its pin UP and its doing this it tells me that your track is somewhat high and angled toward the thumb. I know this because I used to have the same problem with just one ball that was drilled pin high up above my fingers. (i could be wrong but its what was happening to me)

the only thing I can think of is either the ball needs to be drilled to move the track away from the thumb (if possible) or when playing inside like that you need to make sure your hand position ends with your fingers and thumb at 12 and 6 o'clock OR the fingers even more to the right (for a righty) as this will usually push the track straight instead of toward your thumb. (if you keep your hand where your fingers are in toward your body and you dont straighten out during your release then the track will most likely be toward the thumb..... the higher the pin placement the higher your track moves up!)

but as I said this is what happened to me so it might not be your problem but its my best guess.
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tournament average: 219

tweener
medium revs
medium speed
specs and ball layouts in profile

currently throwing
Visionary (O.P. / O. SS. / Glad S.)
Rotogrip (rogue cell)
lanemasters (black pearl)

Is your thumb clearing cleanly? If my thumb is slow to release, it causes my wrist to cock a little and I get oil rings across the thumb hole. It happens early and not down the lane some. Like the first 2 rings.
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whoever said size doesn't matter never had trouble with a thumbhole