I use a custom skin in Minecraft Bedrock Edition for Windows 10. Using the skin is not a problem, I can use it in singleplayer, multiplayer with friends, and multiplayer on servers. However, every time that I close the game my custom skin resets to the regular default skin (Steve).
I've seen multiple posts about how this has been resolved in 1.12.0, however, I'm using 1.12.0 and it still occurs.
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You can bypass the "Only available on specific platforms," message by clicking on your world - multiplayer and toggle off the multiplayer but it still CHANGES the skin back to Alex or Steve. Hope it gets fixed soon!
https://www.reddit.com/r/Minecraft/comments/cc67wp/minecraft_bedrock_edition_keeps_reverting_custom/