Hi @juankpg ,
Given that it is intermittent it may be a loose display flex cable connection at either end, display or systemboard or perhaps it is an internal problem in the display itself. Worst case would be a problem on the systemboard.
As a DIY repair all you can do is to open the tablet and check the cable connections are secure and if they are is then to temporarily replace the screen and check what happens.
Here’s a [https://www.youtube.com/watch?v=3TMh_pKVwRY|video] that shows how to replace the display assembly in your tablet.
Replacement screens are available online. Just search for ''Galaxy Tab S6 LCD screen'' to get results for suppliers of the part that suit you.