I was trying to create clothes for my female character but when I create mask and extract it, parts of torso does not get extracted properly and some parts get joined to the neck, here is how it looks https://imgur.com/a/NokEJoY
Welcome 
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hmm this looks like you have any modifier applied to you model??? As far as i know the mask modifier helps to hide some parts which are intern… so if you wanna select some parts of you model to make clothes the usual way is to copy it make it a new object, add a solidify modifier or using shrinkwrap modifier to model on the object… but that’s just two possibilities…
I have applied Mirror Modifier, Boolean Union Modifier, Decimate Modifier, Subdivision Surface Modifier to my character
Ups i wrote applied but mean addded… my fault… if just added any modification with mask could possibly change the geometry in an strange way. So you have to apply the modifier stack to get final geometry… and youmay rethink your way of modeling a charater but that’s an other story…