Mastery Page bug in New Client

I was starting a new game as thresh support vs a blitz support, and I wanted to change my masteries accordingly for every game. When I opened my mastery page after I selected my champion, the pages appeared blank and were unresponsive when I attempted to fill them. Switching between mastery pages prompted a save request. I had to dodge the game as I was unsure if my masteries were truly blank or just appeared blank. This is strange because this happened after I played a good number of games beforehand, changing my masteries like so without an issue.