I was trying out Rek'Sai today against bots and experienced the same issue. Just glad I wasn't playing against actual people. I have had this happen once before on another champ vs bots though I cannot recall when or who.
Did not notice any triggers... was using same rotation when clearing the camps when it happened as the previous 8 minutes or so. Did everything I thought of aside from leaving the game to regain my cursor but eventually returned to playing half blind and the issue remedied itself half way through a jungle camp a few minutes before the game was over.
Attempted resolutions:
Tabbing in, out, around the tree, and through the hole: cursor would return as long as I was not in LoL.
Changed it to full screen from borderless - did not resolve.
Messed with graphic qualities, aa and vsync - all with no success.