It should be fully functional. What are your device details? Os and version
edit: Oh and are you tapping the "Share" button instead of the "Share as image..." button? The "share" button might not do anything if you're running Voyager self-hosted without a valid SSL certificate or insecure because navigator.share requires secure context.
I am running it on Fennec android (Firefox fork) and valid certificate. I do see the hare as image button. It pops up the menu and you can use radio buttons but share button does nothing.
Nice fallbacks are always good haha. It is a shame firefox Mises out on so many. Pretty neat feature though didn't think one could create a picture from HTML in JavaScript lol