Back glass adhesive coming up

Since the back is glass rather than plastic, I assume the back isn't suffering from being deformed. I'm guessing you've also verified that the frame isn't bent either (if not, check that for sure), so what else would cause the rear to come off?

I've never had a problem with properly applied adhesives, and it sounds like you did it all correctly. The most likely cause for a screen or rear cover coming off is expansion caused by a swollen battery; obviously you'll want to check that if you haven't already.

So outside of that, I'd say the next thing is to make sure you're using a quality adhesive like the one sold here on iFixit.

Galaxy Note9 Rear Cover Adhesive

If you're already using iFixit adhesive, you might contact the iFixit store people regarding your experience; firstly they will almost certainly get you a replacement, and second, they will be interested in knowing if the parts they're selling aren't working as they should.