If you select a Custom Skin then it will show you that you can either pick your old skin on Steve's body, or the new skin on Alex's body. When you pick Steve it just keeps your old skin and body type. When you select Alex it uses the new skin on Alex's body, however, if you pick Steve, then it will use the correct skin if you close and re-open Minecraft. It will sometimes show Alex on the main menu though. Sorry if this is confusing! I can't find other bugs exactly like this so I reported it. Sorry if it was reported before! This is really annoying so a quick fix would be appreciated! <3