iPhone 11 screen dims randomly

Hey, so I recently started having this problem: a short while ago i purchased a new iPhone 11 and the first few weeks everything was totally fine. Until a few days ago, the screen started dimming at random times by itself. I have every setting off, that could affect the brightness, such as True Tone and Night Shift. i also tried experimenting a bit, by shutting off the phone and turning it back on. the first few minutes everything is fine but then it dims again and after like 15-ish minutes it goes back to normal. i also made sure that Low Power Mode is off and that my phone is unplugged from the charger. But this problem keeps occurring. If anyone has/had a similar experience and knows how to solve this, than I’d appreciate a reply very much :)