Possible Rift Herald bugg
I am not sure if they changed rift this last patch or something but 2 times today when the rift got below 300 health it would not open its eye. And it has only happened today. The last time I went around it for nearly a min trying to finish it off. I was forced to dash over the wall on Sej to try and get away only to circle the baron pit to die to rift and its eye still wasn't open. it has only happened today so I'm not sure if they changed something and didn't put it on the patch notes or what.