It seemed this issue was gone for me when I upgraded to 1.5.10 but today it randomly came back when I was experimenting changing some colors (but never saving) via the GUI. I had no borders, then once I clicked "Revert" after looking at different accent colors, I had borders.
Here's some screenshots of what I'm talking about specifically. Around certain Windows windows there is now a black border, but on windows like the File Explorer the border color matches the accent color so there appears to be no border. See?
Lastly, it seems like on certain windows it draws an opposite color b