Yes, that's correct. For example, their messages look like <FONT>example message</FONT>. Despite this, I don't see any actual alteration to their incoming font (no colors or face), it just looks plain. To be honest I prefer not to see the color they use for their fonts.
This only happens with one contact - but it's the only contact that I have on Jabber that uses a custom font, so I can't be exactly sure if it would happen with everyone. It doesn't happen with my other contacts who don't customize their fonts.
They are using Adium. They're the only one of my contacts that use Adium, also.
I am currently using the stable version of Miranda NG.
Yes - this problem persists in a newly created profile.
Edit: Interestingly, after creating a new profile to verify that it does in fact occur in a fresh profile, I returned back to my original profile and the <FONT></FONT> no longer occurs in the original profile...