Problems with UVs misaligning between Painter and Blender (UDIM workflow)

Hi there,

I'm new to Substance Painter and I am having an issue where my UVs are changing between Painter and Blender - it appears Painter is changing them but I can't see where or why.

I unwrapped my model in Blender using a UDIM workflow, and then exported that as an fbx. In Painter the mesh appears correctly and is texturing fine. 

But it appears to have changed the UV map, because when I export those textures (or even the mesh itself) and open them in Blender, they no longer align:

Should I be importing / exporting the mesh differently to preserve UV maps? Should I be using .obj instead of fbx and if so how can I change this without losing the work I've done? Or is there a way to export the new maps that I could then update my mesh with? (noting that when I export mesh from Painter it seems to keep the original UV maps and so is still misaligned.)

Please help! I've been trying to figure this out for a week now and I am completely lost.