I posted here since it's not a technical issue ...

I make a hand after a reference image, extruded the middle finger and after that converted back to poly from subd and duplicated the hand in order to duplicate the finger ...

Well I got the other finger, reshaped it and poly combined it with the hand ... but every time I merge the verts I get non main fold ... every single time ...

I double checked, no double verts or edges or anything ... I tried with border edges on and even I merge all the vers the border edge is still there ... it's not disappearing.
Note: geometry fits perfectly ...

any idea what this could be ?