When using ETF specifically on 1.21.4, it doesn't overwrite the etf_skin_uuid cached skin properly and instead uses it and doesn't allow any customization due to that. Adding/changing any ETF feature changes your cached skin and after changing skins in the launcher and loading back up ETF will just use the cached skin and wont override it no matter how many new edits you make to it, and wont load the correct skin unless you delete the cached file or disable player skin features.
You could probably test it easier by changing the cached file directly instead of changing skins each time. AppleJuce thought you might not have noticed since you didn't update your skin in this version. Maybe Mojang messed something up or changed something? Maybe ETF doesnt update properly?
This issue does not occur on 1.21.1 and 1.21.3. Havent tested other versions but I'm assuming its just 1.21.4.