https://youtu.be/FZmIToR3RSM
This was done in carpet mod 1.4.12, carpet mod as the only mod.

Mob spawned with spawn eggs in this state will show up on /say @e , but not tick entities. If a mob is left behind by running /spawn list, they will show up in tick entities. This is fixed by relogging, upon which the affected mobs are visible.
This also affects all tile ticks, which will not be executed.

this seems to only occur for /spawn list ~ ~ ~

mobs might not be correctly removed when using spawn list, is that right?