20w18a vanilla server

Two situations:
1. Only Overworld is loaded. Cave spider falls into the End portal and gets teleported. Player uses the portal. The spider is gone.
2. There is another player in the end standing on the obsidian platform, so both Overworld and End are loaded. Cave spider falls into the End portal and gets teleported successfully.

The only case when mobs should disappear as described in situation #1 is when there are other players in the End far away from the obsidian platform, so the mob appears there and instantly despawns. However there were no other players in that situation. I can assume that when the player entered the portal, chunks were loaded before the player appeared, though, it's strange anyway. Worth mentioning that nether portals work totally fine.