Had a Pet mimic (gold) on follow, used a waystone.
When i arrived at my waypoint mimic was gone, warped back Mimic wasn't there either
Checked my bed location and the Server's Spawn area for good measure and it wasn't there either
Mimic nowhere to be found...
(Searched on the map with Surface entities only option disabled)

Checked server console and it said:

UUID of added entity already exists: PCChestMimicPet['Pet Gold Mimic'/1497435, 1='ServerLevel[SERVERWORLD]', X=-4559.50, y=71.00, z=5057.50]

I'm guessing the server though it already existed and decided to eat it.

Might be something worth looking into.