I'm sorry in advance if this is the wrong place to post.

I have a Galaxy s5 that previously wouldn't upgrade past Android 6.0.1, so I flashed the phone with TWRP and uploaded Lineage OS 8.1 (which I thought would mimic Android 8.1 but it turns out I like Lineage a lot more). I'm still new to alllll of this, I just followed a guide I found online in order to set all of this up.

My issue is explained in the title. I have the screen timeout set to 30 seconds. Right at the 25 second mark, it dims the screen, makes everything a greenish hue, and then starts flickering. I tried to look up the issue and the only answer I can find is to turn off the adaptive brightness, which I've done. I've also turned off the reduce power usage as well as ambient display and it still does this weird flickering. Is there a known cause/issue for this sort of thing, and if so, what do I do?

Many thanks in advance!