Originally Posted by SirKnowsALot
I would suggest that you keep trying to change pages during selection, and only lock in when the page successfully changes. Or have I misunderstood the bug, and it says it changed but it didn't really change?
If so, sounds like something Riot should take a look at.
It says you've changed it when you're in the lobby but once the game loads it applies a different set of masteries. And as far as I know it's random when it occurs, but once it happens in one game it's going to keep doing the same thing in every other game until you restart the client.
I've also noticed that it switches to the mastery page you have selected in your summoner profile, so it sounds like it has something to do with the game not saving your lobby choice and instead defaulting to that one for some reason.
This is a significant bug and very reproducible, not going to risk playing again until it is fixed.