When "Open browser plugin links in Browser pane" is enabled in settings, opening a plugin link in the Android mobile app does nothing. It should at least try to open in an external browser unless and until Legend supports a browser pane in the mobile app.
When that setting is not enabled, I notice that Gmail links do open externally on Android mobile (although the mobile GMail version won't load the specific conversation so not very useful) BUT Shortwave links still do not open at all. This used to work which was extremely useful and that's why I have classified it as a bug.
I realize you are planning to integrate Vella and I think that's great and am totally open to switching to it (I've actually moved away from using Shortwave due to feature regression), but I do have a lot of links to Shortwave emails in my documents that I would like to be able to access on mobile, so it would be super helpful if I could open the Shortwave links on mobile.