How do I set the default formatting for boundaries on MM9.5 Mac?

I'm creating a template for new maps and I can set the default formatting for the main topic, first-level, second-level and third-level branches, and call-out topics.

I'd like to set a default format for boundaries but when I select a boundary and go to the Format menu and select "Set as Default Format", the settings aren't saved. Is it possible to set a default format?

As a workaround, I'm leaving a sub-topic with a boundary on the template so I can copy this boundary when required.