Weird mesh behaviour while weighting

Can anybody help me with this weird artifact happening on my armatured mesh? I’m trying to paint / fix weightmaps but I always get to the point where there is this ghost mesh happening… To clarify: i have armature modifier set to be visible in edit mode. And the ghost mesh is visible in both weight paint mode and object mode but not in edit mode. See the pictures bellow:

I think i got it, can it be because of non-manifold mesh?