Surface's "line width" property has been an IntProperty since the beginning.
Since December 2014, T18528 (Hackfest: make Core OpenGL free), this property has been redefined to a float value.
This change is very welcome, but it breaks compatibility with older scene files that stored an int value.
I created an example scene file that demonstrates the problem (attached).
A fallback as already integrated for PointSets can be easily added (attached patch). However, as Caspar Goch pointed out, we should see if we can propose a way common to all Mappers to handle these kind of compatibility problems. I'll try to propose something.