Upon loading an area, such as returning from nether to over world, or player log out and back in in a LAN game, there is a chance neutral entities in the area disappear permanently.
Edit: including armor stands, this is also an entity. In both instance, it happened during day time, not sure if mobs are removed too.
Linked issues
Comments 5
This also occurs if you go to far away: summon armor stand in non spawn chunks, unload the chunk by leaving the area, come back and the armor stand will be gone!
Does MC-65040 describe your issue ?