I did a bit of googling, and found out what the function of 'Panel Self Refresh' is. It is a power-saving feature, which uses a kind of extra cache memory for the case where there are static elements on the screen, then the data is sent from the cache memory instead of the main graphics memory (if I understand correctly) which saves power. Apparently this does not work well on our laptops. So we are now sacrificing battery life to circumvent this issue. The question remains:
Does this occur only on our (mine, and the user on the Intel forum LG000) laptops (if exchanged under warranty, does the problem disappear?) or on all XPS 9380 FHD 13's? Maybe also on the 15-inch version? Maybe on ALL laptops using Intel UHD 620 graphics? Maybe on ALL laptops using any Intel graphics with Panel Self Refresh?
I would like to know if the problem occurs on other laptops of the same type. If not, I might want to use my warranty to get a new one.