Ive had this issue before myself, and its usually the threads is where the weak point is. As for the bags, Ive used the vacuum sealer on them, and then double bagged them with both bags being vacu-sealed, make sure you use desiccant in both layers. Its what I had to do to finally make it work.
Very interested to see the rest of your analysis as well!

- - - Updated - - -

Quote Originally Posted by bacpacker View Post
That's a head scratcher for sure. I'll talk to the electricians at work and see if they figured out what caused their problems.
I ended up just sealing them and then coating the seam with waterproof caulking, all I had to do after it was said and done was cut the caulking away and didnt have an issue after that.