@nintendork07
I’ve found a quite simple, yet still temporary solution to this, as I watch youtube in enormous amounts on the daily. When you’ve got the toolbar in youtube fullscreen, click Command+F on your keyboard (the search function). When you X out of that search function the toolbar disappears from the full screen video. When you minimize it returns, but, I’ve found this sequence much easier than clicking up to View and disabling the toolbar completely.

Still having this problem. Clicking "View" and "Hide Toolbar in Full Screen Mode" solves it temporarily but the toolbar reappears whenever the video is exited. I know some people are saying it "has fixed itself" but for me it is definitely still an issue.