Flash and Hexflash get greyed out if you Flash+Ult with Skarner [GAMEPLAY]

So I was playing Skarner, and I flash ulted and then after the cooldown was up and I was supposed to be able to use Hexflash I couldn't. Even after Flash was off cooldown I still couldn't use it as you can see in the picture in the link. It was only usable once again if two conditions are met: 1. Flash has to be off cooldown. 2. I have to ult while Flash is off cooldown. Only then was I able to use Flash or Hexflash. I tested it out later in the game and tried to use flash by itself and Hexflash was usable and once Flash was off cooldown, it was also usable. I went into practice mode and tried Hexflash+Ult and it didn't affect anything, but Flash+Ult made the button unusable once again.

If it's greyed out and you try to use it, it just says "You cannot cast that right now!"

Here are some screenshots of the greyed out.