Hi Mahlon,
I put together a quick character function in an attempt to replicate this.
Even using this character function to insert the activity comments and navigating to the activity via the opportunity, I am still unable to replicate this behaviour. The HTML displays as it should with the exception of “ ” within the blank div tag, for which I have already created a ticket.
With regard to the extra line being displayed, I recommend that you instead use a conditional statement and 2 different character functions. 1 for when a BillAddressAddr3 exists and 1 for when it does not. I also recommend separating these using “<br>” instead of nesting them in a div.
I hope this helps.
-Audisho