Thanks for your report.
The fact is that this "break" feature is still beta and was only implemented for "repeat" loop (and not "while" loop).
Anyway, adding this feature to while loop was not so hard, so we did it in last beta http://beta.plantuml.net/plantuml.jar
We've upgraded the online server so your example is working now (use ctrl-F5 to force cache reload) http://www.plantuml.com/plantuml/uml/HSun3i8m30NGtQVmc5wX5Q8IB2nCN20bloGYJQhQLSNjSLXOREdxIT-ZgDzqNp8TCMMucx3d4q_dZfEm-q0wOkwlbQKkq9ZAQx0WpKq6aq-J6b7-zW7HUmreRjhWtxQH1UnARG0bf9bi_XgflsJumliwa6XTQRIB_VC5
However, do not expect too much on this feature. It will work on simple case, but will fail on complex ones.
So we are interested in feedback!