Alvide, Jayri, Borknar, and Sern (the four Dawnguard refugees) don't ever clean up if they die for whatever reason. In case it matters I sided with the vampires and the entire Dawnguard main questline has been completed, including destroying the Dawnguard in their castle.
DLC1DialogueRefugeeCamp hasn't been shut down, which could be what is preventing the game from cleaning them up.
SQV console output:
Spoiler:
- 4 Aliases for quest 'DLC1DialogueRefugeeCamp' (020198F9) -
REF 'Jayri' -> ' ' (020198B9)
REF 'Alvide' -> ' ' (020198B7)
REF 'Barknar' -> ' ' (020198B5)
REF "Sern" -> ' ' (020198F8)
--- Quest state ---------------------------------------------
Enabled: Yes
State: Running
Stage: 0
Priority: 80