Here’s what the settings for a custom user field look like:
If you click a user’s avatar on a post, you get the user card. That works as expected, but if you click the avatar on the user card, you get to /users/USER/summary. The summary page does not have the Custom field on it, but it’s the activity page that has the user bio and custom fields.
What would I expect
I think I’d expect that the BIO, location, URL, custom user fields and whatnot be displayed on the Summary, not activity page. If somehow it makes more sense that it be on the activity page, then I’d expect that the first user page I get to be the one that has the custom fields and whatnot on it (so point people first to the activity page that has the full user information on it).