1. Create a new skin on "Minecraft: Java Edition">"Skins" tab from a 64x64 .png file.
2. Use skin from 1. on "Skins" tab. "Successfully updated your skin!" displays, and "Current skin" displays skin from 1.
3. Launch a game and enter a singleplayer world or a multiplayer server. (Tested: Vanilla 1.21.1/1.21.3)
4. Player skin of self is displayed as wide variant of Alex instead of skin from 2.. Skins of other players and custom player skulls appear as various default skins.
5. In other players' view in multiplayer servers, player skin of self appears normally as skin from 2.
6.  1-2 minutes after 2., "Current skin" on "Skins" tab switches to Steve skin automatically.