Some mobs like creepers or endermans don't despawn on realms causing massive build-ups and occupying the mob cap
Steps to Reproduce:
Locate some natural spawned mobs on a realm
leave the area by about 100+ blocks
go back to the first area
Observed Results:
Natural generated mobs like creepers didn't despawn and are still there
Expected Results:
The mobs should have despawned according to the new rules
Screenshots/Videos attached: Pictures show natturaly spawned creepers after executing the /tp @e[type=creeper] @s. Same applied to endermans.
Notes: The mobs haven't been hurted, breeded or nametagged
Are these naturally spawned or are you spawning them with eggs because the screenshots do not look like the former and I'm not able to repro what you're describing.