pipe popping

With the compression test you are good.
Your leak down tester is junk.
I have the same one from summit, it is ok at best.
The motor runs hard and great?
Why tear it apart, my bet is still on your exhaust does not seal until it gets hot.
Post a video of what it is doing.

My 78 power wagon has some welded up exhaust patch in it.
I made sure it was good and hot when I went to the inspection station because it was quiet when hot.