It's actually intended that you are not able to get the mastery tokens when playing Legend of the Poro King.
http://na.leagueoflegends.com/en/news/game-updates/gameplay/rotating-game-mode-queue-now-live-7
Just as in any rotating queue game mode, we’ve specially tuned Champion Mastery so you can earn points in the mode. You’ll also be able to earn keys for your wins and loot chests for your (or your premade’s) S-, S, and S+ games. Unlike other Champion Mastery-enabled queues, rotating game modes DO NOT reward mastery tokens towards levels 6 and 7.
Now, the only problem I see is, it states that rotating game modes do not reward mastery tokens, but people have been able to earn them in the past few rotating game modes since mastery 6/7 have been implemented. So my guess is that either it's something specific to Poro King even though it states all RGM's, or Riot is just now making it impossible to earn the mastery tokens in all RGMs. Either way, I'm fairly certain it is intended to not be able ot earn the mastery tokens when playing Poro King, since it does state it in the official announcement page for the return of Poro King.