Running FMPA22.214.171.124 AND latest macOS 10.12.1beta2, I can't replicate this anymore.
Thank you for your post.
I am unable to replicate the issue. This is what I have done:
1. Under macOS Sierra 10.12, I launched FileMaker Pro 15.0.1, and created a database file "Test.fmp12".
2. In Layout Mode, I added a Web Viewer.
3. In the Web Viewer Setup, I changed the URL to "http://www.filemaker.com" and clicked OK.
4. I saved the layout and returned to Browse mode. The web site displays.
5. I closed the file, and updated to FileMaker Pro 15.0.2.
6. I repeated steps 2-4.
Since this isn't working for you with a new solution, do you have any plug-ins enabled. If so, remove them.
If you have other applications open, temporarily quit those applications so only FileMaker Pro is running and try again.