This was beause the code was finding the script DLL compiled for the source region as everything is in the same appdomain and using this as the location for the destination script state, etc.
This resolves the regression by passing the proper destination separately from the DLL retrieved.
Probably a regression since commit d7b92604 (11 July 2014).
Added regression test for this case.
At least partly addresses http://opensimulator.org/mantis/view.php?id=7278
We have to do this since we can't unload existing DLLs if they're all in the same AppDomain.
But we can still update the underlying DLL which will be used in the next simulator session.