It doesn’t appear to be the screen as you say it doesn’t happen in normal GUI use.
So to clarify, does it happen when you touch the screen generally, hence the idea of a contact issue?
When you tighten the screws ensure you do not tighten to much as that can cause distortion if everything is not ‘flat’
You may want to check the pogo pins are all movable, too much tightness may disable one so when you press on the screen it may upset some pins.
There are also five grounding contacts. When you remove the display module and reconnect you may find
a) they are not clean and do not create a good contact between display and main board
b) If the screen malfunctions with even a light pressure then it may be a capacitive issue and dampness could be an issue.
Grounding contacts that may benefit from cleaning, also check the spring contacts on the core module are raised enough
on the
Try using the phone in safe mode to eliminated any custom app issues.
As you don’t have an SD card check how much memory you have, even 10Gb isn’t a lot, and check the RAM usage maybe