Cheers, this helped a lot.
So, after some debugging, I dumbed down the issue to:
1. Me using Theme Park (Good Lock)
2. Me using either an old or "APK" (likely, an old one which somehow turned into "APK" after switching to One UI 7)
3. I also found the specific colour variable (in Theme Park) that changes the App Drawer search bar colours (and more), I've seen a screenshot, but it's the one labelled "Surface". Note: I actually think it uses a combination of this variable and another one. Like, in terms of CSS, some sort of color-mix() where one value is "Surface", I'm not sure what the other one(s) is and I don't care enough to figure that out.
When I make no changes to the theme, and save it as... well, not "APK" (idk how to even export it as APK ) it works perfectly fine - like without even changing colours.
Problem solved, thanks for the hint.