I'm no expert but it seems like the parts' assigned position on the texture atlas doesn't change when assets are reloaded. This means that when a new texture atlas is generated by the game, the multiparts will show random parts of the texture atlas.
I have optifabric installed running OptiFine 1.14.4 HD U F4.

The 1.14 build of Vanilla Parts isn't supported anymore, but I suspect that this also happens on 1.15 because of my model cache. I'll look into fixing this.