It appears that our trusty delve companion Brann Bronzebeard is capped at level 38, rather than the intended maximum level 60.
Last week, Blizzard rolled Brann back to level 15, after several players managed to power level him all the way to 60 using their Warbands, multiboxing and even buying Brann Experience through the Auction House! As Brann gains damage and healing per level beyond 10, that adds up to quite a bit of power difference between players going into the new season.
Maybe he's just too strong for his own good?
Following that fix, Blizzard also clarified that Brann would only gain experience from bountiful delves after level 15, though he still appears to be leveling quite quickly within them — potentially too fast, with Delves very quickly being conquered by players within the first week of the Season, though whether this level 38 lock is an intended gate, another attempt at normalizing the companion's power, or just another bug remains to be seen.