some new information;
metashape doesn't really export obj with udim, but the program exports mesh with submeshes. These submeshes get their own materials applied. Painter creates a texture set for each material.
My solution:
edit the obj and delete all lines with mtl (like in newmtl) and save as a new .obj.
That you can import in painter and you get 1 texture set with a number of UV tiles.
Hope this helps
Regards,
René v G