Experiencing the same problem with larger 3D-models on the HoloLens. The model is only visible where the virtual light source shines light on it. (The bright spots on the 3d-model.) The surrounding area just fades into nothing.
Using a model item to change the color didnt generate better results. (tried white, red, blue, green)
The model has been imported to Creo Parametric as STEP and exported as .pvz.
I am using version 188.8.131.5299 of thingworx studio.