In https://www.phpbb.com/community/viewtopic.php?p=13658081#p13658081 this is used:and surprisingly it still renders the rest of the post in italics. However, even worse it rendered all the following posts in italics as well. "Rendered" because I'm very sure this was not the case at the time the replies were made. Maybe one of phpBB's versions (and their different BBCode engines) since then has made this.
In any case, this should never happen - posters should not be able to let their formattings escape their post scope, neither by accident, nor at will.