Product and version (FileMaker Pro 16.0.x)
OS and version macOS 10.11 and 10.12
Hardware iMac 2013
FMP15 and earlier Web Viewer would hold cookies for the browser session between records. FM16 does not.
How to replicate
Use a web viewer to login to any site that requires a login and issues a cookie for the session. Navigate to a new record.
FM15 will hold the same session in the web viewer for all records.
FM16 requires a new login to the site whenever the record changes.
Workaround (if any)
Any option leaves FM or disconnects context of the record and the web viewer. This is a bad idea as using FM data to navigate pages in the site will not work.