Mobs are not spawning in Realms despite difficulty set to Hard

Description:
Hostile mobs are not spawning in our Realm despite the difficulty being set to Hard and doMobSpawning being true. We tested multiple conditions, but no hostile mobs (zombies, skeletons, creepers, etc.) appear naturally, except for one Enderman. Additionally, no mobs spawn in the Nether at all.

Steps to Reproduce:

1. Set the difficulty to Hard (/difficulty hard).

2. Ensure doMobSpawning = true (/gamerule doMobSpawning true).

3. Move 200+ blocks away to check for new spawns.

4. Check different dimensions (Overworld, Nether).

Observed Results:

• No hostile mobs spawn naturally.

• The Nether is completely empty.

• Only one Enderman spawned after restarting the Realm.

• Villagers seem to lag even when far away.

Expected Results:

• Hostile mobs should spawn as normal, especially in Hard mode.

• The Nether should have piglins, ghasts, and other mobs.

Additional Notes:

• TPS is stable at 20.0, so it's not a lag issue.

• No plugins or datapacks are installed.

• A zombie farm exists 6 chunks away, but even after leaving the area, mobs do not spawn.

• Using /kill @e[type=!player] does not improve spawning.

This issue makes survival gameplay unplayable, as mob farms and natural encounters do not work properly.

Please investigate and fix this issue as soon as possible.