Option f12 or selection from lower right panel works only once.
After the first testing of a page, the test page never changes unless you quit Safari and Dreamweaver and restart both...
Thanks for reporting.
Are you facing this issue with latest Dreamweaver 19.0 as well.
Please check and let us know the steps to reproduce the issue, if it is still present.
What kind of document you are working on, is it occurring for all pages that you are creating in Dreamweaver.