I have been experiencing this too, with dropped items and mobs disappearing . When I follow them through to the linked portal they disappear, but after quitting then re-opening the world they appear back where expected.
While testing in my survival world (and a creative copy of it) I have noticed this to only occur consistently when entering a portal from the nether not when entering from the overworld. I tried moving the affected portal in both dimensions and removing all other nearby portals in both dimensions and after doing both of these it began to work as expected. I believe it could have something to do with having multiple portals nearby causing the entities to not arrive properly in the correctly linked portal until re-opening the world.
Browniie
I have been experiencing this too, with dropped items and mobs disappearing . When I follow them through to the linked portal they disappear, but after quitting then re-opening the world they appear back where expected.
While testing in my survival world (and a creative copy of it) I have noticed this to only occur consistently when entering a portal from the nether not when entering from the overworld. I tried moving the affected portal in both dimensions and removing all other nearby portals in both dimensions and after doing both of these it began to work as expected. I believe it could have something to do with having multiple portals nearby causing the entities to not arrive properly in the correctly linked portal until re-opening the world.