Mastery for skins not working?

[smallspice]

I recently beat the game for the first time and was playing as huntress so I was really excited to get the mastery skin, but for some reason it didn't give me the achievement? Just curious if I just did something wrong or if this was a bug. I was playing with the artifact of command, beat the boss, escaped, and unlocked Captain from that achievement. Is this just something that happens the first time beating the game?

[Blue Goblin]

Were you on Monsoon difficulty?

[Lunacy]

And if you WERE on monsoon, I think going the mithrix route might not unlock it for some reason.

[smallspice]

Thanks, I was just talking to my friends as well and they said that I needed to be on monsoon difficulty which I didn't realize. I thought the wording was to beat the game or obliterate on monsoon meaning that if you beat the game it didn't need to be on monsoon. Either way, case closed so thanks very much.