First of all, there may be some issues with your install of Whonix, seing how the changes from 12 to 13 required a few manual adjustments to operate properly, which likely weren’t done if you just ran “apt-get update && upgrade”. Thus, it might be a good idea to consider a fresh install. Furthermore, Whonix is based on Stable because of the following:
Debian stable receives security fixes faster than Debian testing. For example, by 12/15/2016 Debian jessie was Debian stable and Debian stretch was Debian testing. CVE-2016-1252 was fixed in Debian stable but not in Debian testing, see Debian security tracker by 12/15/2016.
Have a nice day,