Automatic weights wrong for separate - merged pieces

Hi, i have a character that i want to modularize, so i sliced it into parts.
I already had troubles binding an armature to the pieces, so i tried to join them.
But the automatic weighting still generates bad weights:


Although the vertices of neighboring parts are snapped to each other; meaning they are in the same position.
I haven’t merged the vertices as the parts still contain uv maps that i want to keep.
Is this a bug or working as intended (why if so) and/or can i work around it?

Blend file will be good to see. Blender weight paint is a bit shaky when doing character piece to piece when i put my character into module i usual hide some bones when i want to skin part some other part, so let say i want to skin the leg i hide whole the upper body and anything is on top of the leg then i skin the leg and so …