I found a bug, when the player sits down the sitting animation doesn't appear correctly in the minecart or when you ride a horse, it also happens in mods where they add chairs.

I tested it with the latest version of the mod "MorePlayerModels+ v2.1" only with forge 10.13.4.1614.

Have a nice day! ^^

Yep, I'm aware of this issue! It will be fixed thank you :)

Closing! Patched version will be released soon!