I don't know if this is a bug.
While selecting files by holding the left mouse button, when I release it the selection rectangle doesn't always disappear. It disappears when selecting again though.
This happens in Plasma 5.6, 5.7 (didn't test older versions) only on a desktop set to folder view mode or on the folder view desktop widget. Doesn't happen on Dolphin. It also didn't happen on an AMD graphics card with oss driver or proprietary driver. Happens on NVidia with proprietary drivers though. (Didn't test NVidia free drivers).
Some help?