I am using the latest beta and just noticed this issue. I didn't downgrade to check, but I've worked with the same file in which I found the bug before and I know I've done this action. Can't see how I would have missed it if it already existed
Anyway, the bug happens when expanding a multiline selection downwards. It seems as though if the beginning of the selection is sufficiently far away, Sublime Text removes the selection highlight altogether, even though the status bar says characters and lines are still being selected.
Open a large file with >200 lines or so, hold down shift at line one and hold the down arrow key to begin a selection. Eventually, around 100-120 lines selected, the selection highlight will vanish. This also works with all other selection methods when moving downwards, like holding shift and pressing Page Down. If the minimap scrolls at all while the document highlight has vanished, it too will lose the selection highlight.
Expanding a selection upwards always works though, I assume because the "beginning" of the selection is always visible. I guess I will be doing it this way for now