Hello.
At level 15 and higher, Brann only gets XP for seasonal progression from bountiful delves.
We have a hotfix coming that will make this more clear in the tooltip. From level 1 to 14, Brann gains XP from all delves, and then from level 15 to 60, the tooltip will read:
Complete bountiful delves to increase Brann's power in combat during the season.