In the html of a post I have added divs around some content that I want styled a specific way. However when I come back to this post later, these divs have changed to paragraph tags and the closing tag is moved. The div I created encompassed four paragraph tags and an h4, but when I come back it's moved so that only h4 is in between (this was the first tag in the div). In fact this change will happen if I switch back to the text editor or go to any other tab.
This is a bug with the core wysiwyg editor not ProBlog. That said, if you add your div and do not switch back and save/update, you should notice the div is actually there. Of course, then editing that bug converts it to p tags.
All that said, you could simply style the p tag instead of a div as the class does still seem to remain. (not ideal, but certainly doable)
I will mention this post to the core dev's and see if they have any ideas.