Thanks for sharing your investigation.
I'll keep this in mind if someone will have a such problem.

But in fact, there is nothing to fix. The URL is valid.
Perhaps for Hololens it is required that the response had some specific header.
Such a problem exists for Bing Maps on WebGL.

After investigating some time about my problem in the Following topic : http://forum.infinity-code.com/viewtopic.php?id=890

I finally found that maps were not loading because the provider was Nokia. Changing it to anything else worked properly.

This bug only showed up when Moving from unity 2017 to 2018.