I am also having this issue. I am on windows 10. Just finished building my aquarium and was ready to fill tanks with fish but they all turned white. The name even changed to white.
mobs are disappearing even if they are tamed, named, interacted with, inside a pen, bred, traded with, and on leads. Here is a list of the mobs that have despawned in my realms world playing on PC windows 10 version.
many dyed sheep that were bred in pens vanished
tamed horse with saddle attached to post with lead
many tamed horses with saddles in pen
A named polar bear in a pen
foxes that were bred in a pen
ocelots that were bred in a pen
multiple captured villagers that were traded with to max level and named
a named endermite in an enclosure
This has been very frustrating as many resources were spent on these mobs such as name tags, saddles, dyes, and emeralds. Also spent hours traveling thousands of blocks to obtain some of the animals. nametags are expensive to trade for and difficult to find in world especially since all areas explored that are close to spawn we have already looted. I want my nametags back and my jungle friends as the jungle was so far away to travel to and took a lot time to bring them back to spawn
Here are some things that I have noticed in regards to my mobs disappearing hopefully this helps to find a fix.
im playing on windows 10 realms
the newer mobs seem to disappear at a much faster rate. for example the original 8 horses we had took months slowly disappearing one by one. However recently I bred a large amount of sheep knowing some would disappear, however within 3 days every single one was gone. Have had this experience with the rate of other mobs vanishing as well.