Hmmm yeah clicking one of those popup for different hexes is required to actually cast hex. Otherwise the game thinks you're cancelling hex to do a melee (or ranged if you have a bow equipped. Trying this would be a good test) attack.
I'd recommend trying to cast hex without Chromecast/remote, directly from your pc, to see if you can click on the hex popup boxes that way. If so, the problem is an interaction between spell popups and chromecast/remote.
Otherwise, if this bug is present during direct and remote play, then it is a bug with Hex for some other reason.
Either way, you'll have more knowledge to submit a proper bug report to Larian using their feedback link
Hope this gets fixed!