Hi Nicole!
I was also in need of changing the welcome message and got confused by the unfinished tootip. I found an official bbb page that (in the "welcome" parameter) writes "The welcome message has limited support for HTML formatting." `I also found this topic and I realized that a message should not exceed 255 characters.
From all the tests I tried, I can confirm that the %%CONFNAME%%, %%DIALNUM%%, %%CONFNUM%% variables work fine, but not any of the html tags. "It will always show in one line without any bold or italic letters.", exactly as you describe.
I still need an answer to this. Any contribution is welcomed