I'm using the latest version available for 1.18.1: 3dskinlayers-forge-1.4.3-mc1.18.2.jar, I have the second layer of my skin disabled but when in first person the second layer of the arm is still visible, in third person it isn't there, without the mod it works as it should

Uhhhh huh. Looks like a missing check, will be fixed in the next release.