Seems logic as the Flash Player always taped into the browser capabilities for many things like sockets, languages, event loop, etc.
I don't see why it would be considered as an issue, it's not a bug it's the wanted behavior:
it is normal to not have access to all the "details" when the browser is in "private browsing".
Look at Font Fingerprinting on browserleaks.com
Font fingerprinting – is what fonts you have, and how they are drawn. Based on measuring dimensions of the filled with text HTML elements, it is possible to build an identifier that can be used to track the same browser over time.
Font metric-based fingerprinting is tightly crossed with the canvas fingerprinting. It is probably weaker fingerprinting technique, since canvas gets not only bounding boxes but also pixel data. On the other hand font fingerprinting is much more difficult to defend.
So when your browser is in privacy mode I would say blocking the listing of fonts would be mandatory.