See thread linked below.
TL;DR version: long-running world has lots of beefaloherd entities with no members (components.herd.membercount = 0). In the attached save, out of 123 herds, 89 are empty.
The snapshots in the attached cluster were, going by the timestamps and server log date, all played in 1 session - however, there are several instances where the cluster was rolled back (timestamp delta between snapshots > 8 minutes), and this is exactly where the number of empty herds increased. I use this "production" cluster for fight training as well, so it has seen a lot of rollbacks over the years.