Hello @i0np
First of all, thank you for your interest in the viewer’s API 2.0! 
We’re always glad to see people trying things out!
The issue you are describing is a valid one, and it’s already being tracked in this issue. API 2.0 is still not thoroughly tested and it’s likely you may encounter bugs like this one
Fortunately, API 2.0 provides another way to achieve what you are describing, by using the viewer-core directly and it’s setMaterial functionality. Here is a working example to get you started ( I suggest you use Chrome to open it, it seems there is currently an issue with their sandbox website on Firefox and possibly other browsers)
Additionally, I recommend you have a look over API2.0’s quick reference guide to get an idea of the available functionalities in the viewer
If you have any further questions, just let me know and I’ll gladly assist
Cheers