This ticket just exists to make sure this issue is known to the Minecraft Realms team.

Multiple users have reported that entities, such as armor stands, item frames, or minecarts, disappear after a while in the world. There's a collection ticket in the MC project: MC-150170

As all occurrences of this happened on realms servers, it's likely that this is a realms-only issue. However, I wasn't able to confirm whether or not this is actually the case.