Tried to reproduce using default cube and sintel models. In both cases subdivide smooth didn't split vertices at all. This could depend on some particular topology. Anyway, having sample file on which the issue 100% reproduceable would be really helpful :)

Antony, as a guy who's really familiar with UV maps in blender maybe you've experienced something the familiar or could check on this?

i had an other look at my scene and noticed that there is a multires modifier on the object. i also tried to reproduce it again and its only possible with the multires modifier.

so to reproduce it add a multiresolution modifier to the mesh with 1 subdivision and smooth subdivide the whole mesh.
i also attached a test scene where the only thing one has to do is smooth subdivide the mesh to get the uv splitting.

Aaaah, sure as the morning sun, there it is again, BM_loop_interp_from_face. I didn't even have to put any energy to it, just added a breakpoit there and it rang as soon as I run the operator. There must be a way to fix this in a way that avoids this offset :/

Should be fixed in r48395.

its back agin in r50409

I can't reproduce here on 50730. Can you provide me with a new blend? Old one behaves OK.