Panagiotis,
You are not the only one who has this problem. I have this issue, too.
I have 3DM-ABS and 3DM-DENT resin. When I printed it took around 4-5 hours and I saw the thin half-cured layer at the bottom of VAT.
3DM-ABS has less issue, but it still shows the issue. 3DM-DENT has worse. I guess 3DM resins are quite sensitive than other resins. 3DM-ABS is half-transparent, but 3DM-DENT is not and it is brown color. I guess 3DM-DENT absolve more UV than 3DM-ABS and has more half-cured resin layer at the bottom of VAT.
I lowed my PC gamma and brightness, that didn’t much help.
When I put a white paper on vat, I realized the projector has quite bright ambient lights in the display frame area.
I tried to low this ambient brightness in the projector menu, no one affects this ambient brightness.
I am looking for a solution how to get rid of or lowed this ambient brightness.