QuadRemesh : UVs get messed up with export


I have a quite simple object, representing a T-Joint for welding.
For a certain technical reason I need to export a Polymesh tesselated to pattern of 2.5mm Quads.

I do the layout of the UVs on the Polysurface, which then transfers well to the Polymesh during the tesselation with QuadRemesh.

But when I export the object the UVs getted messed.
I did the same with a Trimesh created with the Mesh function. That works fine.

Is it a bug?
Are there any ideas how to overcome this malbehaviour?

Thank you in advance,