Display issues 4th gen 5th gen

I have 2 x 12.9’s with the same issue one hasn’t had anything done to it the other had a new screen assembly installed.

Issue is the display is very faint can only see with a light on it. Tried all the typical solutions to no avail, decided to replace the screen assembly put the iPad on the heat mat to remove the screen and the screen comes back to life, I get the “too hot” first when it cools down it’s fine and works normally, few days later same problem, heat up the iPad and it works again for a few days etc…was going to fully replace the screen assembly but now I also have a 5th gen 12.9 with the same issue but the screen was replace A few weeks ago!

Any ideas

Cheers