So I have been trying to get my playing a bit more lateral, so I was using Ben eller's bark at the moon lesson. I notice something I hadn't before.

My thumb is in proper position at " | " the start of the run but by time I get to the end. My thumb has turned and is parallel to the neck "- "

As I watched myself play it was a gradual change from bring perpendicular to the neck to parallel and it would get farther away from my fretting fingers.

I tried wrapping some scotch tape around my thumb and this seemed to make it slide easier. Have I discovered an actual problem? And if so how do I fix it?

I have a schecter blackjack sls.

You might be pressing too hard with your thumb, and the friction is causing it to shift in the same place when your hand moves. The role of the thumb is to provide balance and support for your fingers and should rest on the back of the neck with little to no pressure. If that's the case, try lightening up and see if that helps (which since you mentioned the tape working it should).
cujohnston thanks for the reply just making the kids lunch and the ill give it a shot