So I met the requirements for the Warlock Mastery and Sunbreaker's Challenge achievements within the past few weeks or so, but neither of them have unlocked, even after days of playing the game, signing out, hard resetting, logging out characters, you name it.
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Sometimes they can take a few days to pop up for some reason, so that could be the problem on your titan, but on your warlock, do you have another subclass maxed besides stormcaller? The Warlock Mastery (and the equivalent achievements for the other classes) require you to max out one of the two original subclasses, so you would need Sunsinger or Voidwalker.