Sorry to respond without a solution for you, but I wanted to say that I've had this issue as well and hope that maybe bumping your thread will be more effective than making my own. I'm running the stock Manjaro ARM distro off of the built-in eMMC in a Pinebook Pro that I just received in the mail last week. All software (incl. OS & drivers) is up-to-date as of 9/20/2020, according to the built-in package manager.
The issue does indeed seem to be a graphical problem with colored, highlighted transparencies, and I see it in the exact same place you've noticed it - the Plasma subheading of the settings window. I have not noticed any other graphical issues, relics, tearing, etc. related to transparencies or highlights, in fact I don't know that I've really noticed any other graphics glitches at all. I believe this glitch goes back to day 1, prior even to updating from the stock OS build, but now I'm actually not certain...
Side note - I do want to say that, after about a week of fairly intensive config/messing around, this thing has continued growing on me and in general I've been super duper impressed with it - shout out to the fam at Pine64. Thanks fam!
The issue does indeed seem to be a graphical problem with colored, highlighted transparencies, and I see it in the exact same place you've noticed it - the Plasma subheading of the settings window. I have not noticed any other graphical issues, relics, tearing, etc. related to transparencies or highlights, in fact I don't know that I've really noticed any other graphics glitches at all. I believe this glitch goes back to day 1, prior even to updating from the stock OS build, but now I'm actually not certain...
Side note - I do want to say that, after about a week of fairly intensive config/messing around, this thing has continued growing on me and in general I've been super duper impressed with it - shout out to the fam at Pine64. Thanks fam!