When you using custom skin, and the models is set to slim model, everytime you restarting the game it will changes to classic model
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Steps to reproduce:
Import any custom skin, and then select slim model
Close your game and then reopen it
Expected results:
Custom skin models doesn't changes and keep using slim model
Observed results:
Custom skin models changes to classic model after restarting the game
Workaround:
Until these bug got fixed, for now you can create a custom skinpacks and add it to your games, since these bugs does not apply to skinpacks it should be fine, I've attached a sample skinpacks to the report, here how you do it:
Download
and extract it using your preferred File Manager (i recommended using ZArchiver for Android and 7-Zip or WinRAR for Windows)
Copy your skin files inside the extracted skinpacks folder, after that rename your skin files to slim1.png, slim2.png, slim3.png, slim4.png, or slim5.png
Zip the folder again, then rename the extension from .zip to .mcpack
Import it to your game
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Confirmed

Can be confirmed.

Can also confirm this is happening to me as well. It is especially annoying to have to redo my skin every time I load the game up

Can also confirm this is happening to me as well. It is especially annoying to have to redo my skin every time I load the game up

Can confirm this is true.

It's back again as usual... I hate it because it's time consuming and annoying as hecc.

It's back again as usual... I hate it because it's time consuming and annoying as hecc.

Annoying bug

Annoying bug

This bug is happening to me and a friend!

This bug is happening to me and a friend!

Positive.
Just like described and shown in the video.
It doesn't matter when you leave the game either, so it can’t be that the game needs time to save it.

me too!

me too!

confirmed for me as well on 1.21.44

me too

me too

Happening to me, as well.

Am also dealing with this bug. It's annoying because my skin is only built for the Slim model Arms, so if I use the classic arm model, it has empty gaps all over the skin's arms.

Am also dealing with this bug. It's annoying because my skin is only built for the Slim model Arms, so if I use the classic arm model, it has empty gaps all over the skin's arms.

What did Mojang change to cause this bug?

The bug still occurs on 1.21.51

Still occurs 3 months later.

Still occurs 3 months later.

Still hasn't been fixed I contacted support and they didn't even help.

Still hasn't been fixed I contacted support and they didn't even help.

Surprised this issue is happening for others too. It is getting quite annoying

Surprised this issue is happening for others too. It is getting quite annoying

Workaround:
Until these bug got fixed, for now you can create a custom skinpacks and add it to your games, since these bugs does not apply to skinpacks it should be fine, I've attached a sample skinpacks to the report, here how you do it:
Download
and extract it using your preferred File Manager (i recommended using ZArchiver for Android and 7-Zip or WinRAR for Windows)
Copy your skin files inside the extracted skinpacks folder, after that rename your skin files to slim1.png, slim2.png, slim3.png, slim4.png, or slim5.png
Zip the folder again, then rename the extension from .zip to .mcpack
Import it to your game

For those without access to third-party archiving tools you can do this:
Download the mcpack from the pinned comment and rename the extension from .mcpack to .zip
extract the now renamed file (MCPE-186591.zip) to any location (Ex: C:\Users%username%\Desktop)
Follow steps 2, 3, and 4 of KejuGorenk's workaround tutorial.

Experiencing the same issue, except:
Skin models change to classic upon joining servers/realms as well from what I've seen.
The skin doesn't adjust to the model/changes properly, leading to gaps in the texture. The skins I use display fine on both classic/slim when I use them in Java (instead of only being made for slim), so I'm not sure why that's happening.
The workaround is no longer working (playing v1.21.51); I get a popup stating it's not a valid zip archive. It has been changed to .mcpack and I double-checked by unzipping it that I did everything right, so that's interesting.

whoever patched this issue didnt patch it because since february i have been having the exact opposite of this where all of my wide skins will default to slim every time and i need to delete and reimport them whenever i open the game. please either reverse the change or actually fix this bug.

Definitely not resolved, mi skin is wide and every time I restart the game it sets back to slim, wich makes it look awkward since its designed for 4 pixels in the arms.
Every time I must reload de skin file so the game let me select between wide and slim.