This bug has been one that has been around for a while, where usually a cape or an elytra is mistakenly counted as a chest plate, causing the player’s skin to look like how it would if it was wearing a chest plate, despite the fact that a cape or elytra shouldn’t remove those parts of a players outfit. This bug doesn’t affect the customisation screen, since it doesn’t count anything as worn in there. Images linked show examples of the customisation screen compared to actual gameplay
This issue is being tracked at MCPE-161851.