I run a bedrock server , and since the 1.17 update we have had entities despawning. 
First instance was a Chicken cooker farm, 20 chickens were nametagged and led to top containing area where eggs are collected.

server command stop to shutdown the server.

Bedrockserver.exe ran to start a new instance of the server.

Upon logging in the chickens are gone and cooked chicken can be found in bottom collection chest.

Server upgraded to recent release 1.17.11.01

within 1 chunk, 8 individual sheep shearing farms were set up. 

Sheeps were name tagged and colored.

server command stop to shutdown the server.

Bedrockserver.exe ran to start a new instance of the server.

Upon logging in three of the eight sheeps are gone and cooked mutton can be found in bottom collection chest with the wool.

Admin in creative mode, respawned sheep, re colored, renamed tagged, so entities was back up to 8, same server restart and was able to reproduce the issue. 

update 09/16/2021

after servers had issue connecting to Microsoft account services, and services were restored more of the sheep were killed/despawned with cooked mutton in chest, upon the server being restarted again.