Weld Command and UV map

[micheus]

The "problem" is the UV map coordinates is a vertices attribute, not a material one. So, by welding you are removing one of the two matched vertices from a face (like in my sample) - I think that could be the problem (but I can be wrong)

You probably will not smooth the object, so if you don't mind to have some extra and tiny faces close to the weld region then you could try to extrude a short amount the face that will be welded. But, use the Weld [RMB] and set the tolerance to a shorter value.
I'm not sure it would work, but you could try.