Hey,
So far I didn't find any pattern, it appears to be random. The guid is the blob id generated in the JS code that handles the download of the file.
When I get the file name everything is correct and as expected.
And yes, the file name always contains the expected file name. In my sandbox I even hardcoded the file name.
I've heard that the React 16 preview may solve the problem, which is something I'll test tomorrow.
Still, I'd expect the download to work even without the React 16 preview.
Cheers