The skin is used as a cape

[Tartilla-TAIGO]

MineCraft versions: 1.12.2 (Vanilla), 1.16.5 (Forge) and others maybe
When using a cape, not the texture of the cape is used, but the texture of the skin, upon request, the texture of the cape is returned to the skin server

[11:26:52] [Client thread/INFO]: Setting user: test2
[11:26:53] [Client thread/INFO]: LWJGL Version: 2.9.4
[11:26:53] [Client thread/INFO]: Reloading ResourceManager: Default
[11:26:54] [Sound Library Loader/INFO]: Starting up SoundSystem...
[11:26:54] [Thread-3/INFO]: Initializing LWJGL OpenAL
[11:26:54] [Thread-3/INFO]: (The LWJGL binding of OpenAL. For more information, see http://www.lwjgl.org)
[11:26:54] [Thread-3/INFO]: OpenAL initialized.
[11:26:54] [Sound Library Loader/INFO]: Sound engine started
[11:26:55] [Client thread/INFO]: Created: 1024x512 textures-atlas
[11:26:56] [Client thread/INFO]: Narrator library for x64 successfully loaded
[11:27:04] [Client thread/INFO]: Connecting to node.taigo.xyz, 20000
[11:27:06] [Client thread/INFO]: [CHAT] test2 joined the game
[11:27:06] [pool-2-thread-1/INFO]: [STDOUT]: https://dpc.launcher.age-craft.ru/api/skins/test2.png
[11:27:06] [pool-2-thread-1/INFO]: [STDOUT]: https://dpc.launcher.age-craft.ru/api/capes/test2.png
[11:27:06] [Client thread/INFO]: Loaded 7 advancements
[11:27:07] [pool-2-thread-1/INFO]: [STDOUT]: https://dpc.launcher.age-craft.ru/api/skins/test.png
[11:27:07] [pool-2-thread-1/INFO]: [STDOUT]: https://dpc.launcher.age-craft.ru/api/capes/test.png

[Tartilla-TAIGO]

If someone has the same problem, then the solution is: you need to use different names in the download link for the cheat and skin file, because they are stored together. It is best to use the hash of this file.

The solution was provided by: yushijinhun