simplest way to reproduce I could find
/tick freeze
/tp @s ~-2000 ~ ~
/summon mannequin
/kill @n[type=mannequin]
/tp @s ~2000 ~ ~
/tick step 2should generate something along the lines of
10:33:22.060 emy Server thread [emy] Serialization errors: chunk@[-125, 0]cvl['Mannequin'/227, l='ServerLevel[New World]', x=-1993.50, y=-60.00, z=7.50]: Failed to encode value 'DYING' to field 'pose': Invalid pose: dying
Not sure what the implication is, but until I force load the chunk or teleport there I keep getting the errors on every automatic save, which makes me wonder if maybe that implies that the server refuses to unload the chunk fully?
Can confirm 1.21.11
Any sort of saving causes the serialization error.