Rek'Sai Tunnel Doesn't Die

My last game just now, I played Rek'sai top lane versus a Nasus. At some time I created a tunnel that didn't disappears upon being sat on, or after set timer. I only noticed it after I destroyed Top inner turret when enemy jungler tried to break it and it wasn't even debris rising. So at that point the tunnel simply existed all game, enabling me to ultimate to it at any time I so desired. I believe that ultimating to it at last second of its life or while it's being destroy may bug it to permanently keep it alive. That is only a guess. I'm not entirely certain what caused it, but it definitely exist.