In my modded survival I'm dead in the nether with my Backpack (with the piglin-skin) and when I retake my stuff and change my backpack skin, the piglin one was locked... and the count of "nether biome explored" disappear...

What version of the mod are you using?

2.1.2 (the latest version available in 1.18.1)

(Sorry, if it was fixed in a newer version... but if there's a way to get the backpack back, that would be great...)

Yeah, it is fixed in newer versions. Backpack should be available just by reloading your world.