This is critical bug that makes default or Standard skinpack doesnt work for multiplayer (Tested on server, world, and realm with 2 devices) With this bug, you can only use skin that came from persone skinpack. So if you use "Steve" and "Alex" that's from default skinpack, the skin is just invisible as you can in the video below :

https://www.youtube.com/watch?v=QRzl11g0k4Q

*The video is up to 60mb, so i cant post it here...