No problems for me with the Creators Update.
The only way I could get one of my Centaur models into Paint3D was to first import it into 3D Builder (via wavefront obj), allow it to 'fix' the model geometry, then save as 3mf, then load in Paint3D.
It is disappointing that Microsoft didn't provide a Collada DAE import, unless I missed it.
Your model must be UV mapped for Paint3D to work its magic.
The 3mf file is a zip file, unzip it and you will see how Paint3D updates the texture image file.
Both Paint3D and 3D Builder require that the object geometry is a solid - it is important that every edge in the model must be bound to two, and only two, faces. This allows for the 3D printing aspect, which makes sense.
Surface normals are not preserved, so the model will appear faceted, but I'm not sure how a 3D printer would create the model, whether it does its own smoothing or not.