Randomly on loading the chunk, whether it's from a nether portal or just walking, item frames in a certain chunk will despawn. This also makes the items in said item frames despawn. This chunk does not include a nether portal. 

Signs, banners, and anything else seems unaffected. On chunk load, it just has a very slim chance of despawning these item frames. Sometimes it includes multiple chunks that border each other. 

Villagers also seem affected by this, despawning the villagers I brought to my base.

No idea how to replicate, as this has such a rare chance of happening (3 times within 1.5 weeks) that I don't know how I would replicate.