Foxes don't stop moving outside of a 40 block radius around the player, like other mobs do.
I don't know if that's a bug, but it seems like a memory drain to me
To recreate, create a flat desert world and wait a minute.
Then spawn in som foxes to kill the bunnies.
After they killed them all (very entertaining) they will wander about, until they wander out of renderdistance.
Are you playing Bedrock Edition or Java Edition? Mobs should not stop moving outside of a 40 block radius in Java Edition.