Steps to reproduce:

• Have three XMPP accounts, named “Collabora”, “Facebook”, and “will@mydomain.co.uk” (obviously your mileage may vary, but for clarity I'm using the names I have);
• Pick Chat → New Conversation…
• Choose “Collabora” or “will@mydomain.co.uk” from the account dropdown, enter a JID, and hit Chat;
• In the resulting conversation window, verify that the tab tooltip and Contact → Information dialog both show the correct account;
• Pick Contact → Add Contact…

Expected results:

The account I was using to chat with the contact is pre-selected in the dialog.

Actual result:

The Facebook account (which is the second account, alphabetically speaking) is selected. Adding random contacts doesn't work on Facebook, so this essentially renders the dialog non-functional.

I've done a bit of tinkering, enabling and disabling accounts. For any given set of enabled XMPP accounts, the same (apparently arbitrary) one is selected whenever I open the dialog, but I can't see any obvious pattern. It's not always the second, for instance. If enabled, my Facebook account seems to be selected in preference to all others. It's probably just the first account in some hash table or something.