The bug
It looks like the world generation can spawn too many monsters at the same place.
Note: MC-4061 is marked as related since @unknown had one chunk error while trying to reproduce this bug, however that time no monsters spawned.
How to reproduce
Outdated, though this issue might still occur
Ensure your render distance is set to at least 9
Create a default world with the following seed
8627439086671208170
Teleport near the position
/tp 920 70 -1736
Wait some time, maybe about 10 seconds
Switch to Spectator mode
/gamemode spectator
Teleport to the exact position
/tp 908 27 -1774 180 50
Can't confirm in 1.15.2. There was a cave at that place. Seems like world gen has changed a bit