When you uncheck "Use GTK mouse selection" the selection can have odd side effects. To reproduce: Select a couple of items Click and hold down on another item. All 3 end up selected instead of just the last one that you might expect.
Not a bug. That's the way the "non-gtk" selection mode works. It will reselect whatever was prevously selected if you do not release the mouse button or start a drag action. The "non-gtk" selection mode has been removed from version 4.3 since it is confusing to many.