ok
So I think I found where the issue is , and is a bit tricky as you hadn't told 3am you were using QTTABBAR I recognized on your screen
It uses its own color scheme so you have to go to apparence section in qttabbar options > folder view tab and uncheck " color view border" or change for another color
As you can see I could reproduce your issue with red color
let us know if it works
bye !