by Bruno Pagès » Fri 6 Feb 2015 16:40
You are right, if you open a diagram then do some changes then an HTML export, the changes you did are not visible in the PNG/SVG picture for the diagram. It is needed to close the diagram to have the changes taken into account by an HTML export. This is the case for any diagrams, not only the use case diagrams.
Thanks for the report

Author of Bouml