Yes this looks like a bug, I think I see what the problem is, but I'll need to take a closer look to fix it.
We show the fields on all user cards, on the profile or on the discussion list. Likewise, the bio fields relationship can be loaded on both BasicUser and User, so it should be loadable on the discussion list as well.
What's missing is the actual include in the discussion list. So this means that if you visited a user profile in the same browsing session, you will see their fields in the discussion card, otherwise you won't.
If someone can replicate this, feel free to open a GitHub issue explaining the steps to reproduce. Otherwise I'll get to it once I find some time.