Uploading a world to an existing Realm causes visual errors

The Realm owner uploaded a world to an existing Realm, and multiple things happened to them only. It was originally their world.

First, all the zombies and skeletons that were previously loaded before the Realm was uploaded went into a crawling/swimming model, and didn’t render any armor they had equipped. All zombies and skeletons that spawned afterwards were unaffected.

Second, the tropical fish rendered very strangely. Depending on how close you were, they would turn from a magenta cube, a corrupted model with (most often) a zombie texture, and finally another corrupted model with various different entity textures, including a zombie texture and a turtle texture, and for all three different visuals they were unable to be picked up in buckets.

Third, all players visually were invisible, except the items in their mainhand and offhand, and including their armor.

Fourth, all breaking particles were invisible, and any effect particles were black.

Only the Realm owner observed these side effects. I was on the Realm at the same time and did not observe any of these effects.

How to Reproduce:

1. Make a world and spawn multiple zombies and skeletons, and give some of them armor

2. Save and quit the world, then upload it to an existing Realm

3. Open the world through the Realm and look around. The zombies and skeletons should be crawling and no armor is rendered.

4. Spawn some zombies and skeletons, and observe their model.

5. Finally, spawn some tropical fish. They should have broken models.

Note on Screenshots

Resource packs activated: Faithful 64x (CurseForge), Déesse UI Pack (CurseForge), Winter Resource Pack (Custom resource pack that I made, and only modifies block and item textures and colormaps). One screenshot is of a crawling zombie next to a regular zombie (The crawling zombie has a copper helmet), the next is another crawling zombie, the third is of the magenta cube tropical fish, and the fourth is of a tropical fish with a corrupted model and a zombie texture.