So I've looked at this in a bit more detail ...
Firstly, even though I wrote the code, my recollection of how the plugin handles feeds was incorrect.
From reading the code, enabling Display in RSS Feeds in the plugin's Display tab is basically an all-or-nothing setting. If you enable the biography box in RSS feeds then you should get the biography box in all feeds, in other words for the main page feed as well as for the archive feeds and specific post feeds.
There isn't (currently) any finer grained control for feed support; so you can't, for example, enable feed support for a specific category's feed, but disable it for other feed types.
At least, this is what my local testing environment is telling me and this is backed up by the plugin's code.
What I don't understand is why you're seeing the biography box in your main feed and not in your category feeds.
Maybe we should start at the beginning ...
Are you using any of the plugin's shortcodes or template tags?
Have you tried disabling all other plugins and checking whether this recurs?
Let me know how you get on, I'm anxious to try and work out precisely what's going on here.