I have encountered problem with middle edge loop of my model.
Some vertices of that edge loop are snapping to other side along Y axis when I'm trying to move them.
Model was previously sculpted a bit, and I was using mirror and multi resolution modifiers.
I first time noticed this problem when I was weight painting chin of my human, the weight paint was appearing also on the neck of the character. I also checked previous versions of my project, and weight paint has nothing to do with this issue.
On the image I selected whole edge loop and moved it along Y axis, ofc. there should be no gaps between vertices.