To test this, I reloaded my save files from various points in the quest line and each time gave Seranna as many items as she can carry. What I noticed was:
Save file 1 (39h 50m 14s): Seranna could carry 281 weight. At this point the quest Prophet is active at the step "Ask people in ****** ****** if they saw the **** ******".
Save file 2 (79h 15m 19s): Seranna could carry 83 weight. At this point the quest Kindred Judgment is active at the step "Confront ****** with ******** ***".
Save file 3 (80h 49m 06s): Seranna could carry 65 weight. At this point the Dawnguard Vampire-side questline is complete.
Save file 4 (83h 27m 55s): Seranna could carry 64 weight. At this point the Dawnguard Vampire-side questline is complete.
Has anyone else noticed this? Can anyone else confirm?
I'm not sure what's causing the reduction in her carry weight. During the playthrough, she was constantly present as the main character's companion. Generally, I would enter a dungeon/area with Seranna, fill up on loot until that dungeon/area's task is accomplished, then sell/store the loot before returning to the quest giver to finish the quest and progress the story. After finishing the questline, Seranna could barely carry the gear I give her for her use in combat. I don't know if any of these details is pertinent to why her carry weight is gradually reduced.
I hope this can be fixed by Bethesda! Do their devs monitor this forum at all?
Thanks.