Entities randomly appear/disappear on multiplayer server between reconnections.

Meanwhile the server constantly shows warnings similar to these:

[13:40:38] [Server thread/WARN]: Keeping entity minecraft:chicken that already exists with UUID 329fecbe-624c-47b1-8c1c-f3b0531771e5
[13:40:38] [Server thread/WARN]: Keeping entity minecraft:villager that already exists with UUID e901c66d-0a68-4a40-98d5-27459404b081

Steps to reproduce:

• find a village, e.g. at (-459; 77; -925) position on seed 5000000123456
• summon 70 chicken
• summon 20 villager
• disconnect
• reconnect

After this the result is random.

• only 1 or 2 villagers/chickens are present
• none of the animals and/or villagers are present
• all entities are present
• all entities are duplicated (even my horse with diamond armor)
• etc.

Look at this animated gif:

https://www.dropbox.com/s/eq06ij1ryqm0ly7/MC_19w08a_Entities_diappear.gif?dl=0

I tested it with more seeds and more animals, and results were the same.

I also tested with previous versions, and 19w07a was still NOT affected!

I tried to reproduce it in single player mode, but I couldn't.