First of all, I'm sorry if this is a duplicate, this is an oddly specific thing to search for.

I've recently come across a bug, where any passenger of an entity will appear frozen if the entity being ridden is far enough away to stop being rendered, until you relog or the root vehicle entity is killed. Here is an example:

The armor stand on the left is actually riding the ghast on the right. What happened is, I flew far away until the ghast was in a position where the client wouldn't care less about rendering it. After going back to where the ghast was, the two entities were visually separated.

Here is a command to easily test it: (This not only happens with ghasts and armor stands. I've come across this with multiple entities)

/summon Ghast ~ ~2 ~ {Passengers:[{id:ArmorStand,ArmorItems:[{},{},{},{id:obsidian}]}],PersistenceRequired:1b,Silent:1b}

Here is a gfy, reproducing the bug:
https://gfycat.com/HandyAcclaimedCottonmouth

This is a very difficult thing to get around as a map maker. Most of what I do involves passengers and entities, but this a very limiting bug that I hope to see fixed in the near future.