In 2.x the bbcode tag URL worked spanning multiple lines, so you could tag a whole address or an IMG and a description together with one URL.

In 3.x the URL tag does not function if there are multiple lines in the post.

Several other bbcode tags work for multiple lines: b, i, code, quote and the function of the HTML tag <a> plus the functionality of phpbb 2.x makes me think this is a bug that multiple lines do not work in 3.x