No, backports really aren’t upgraded to by default. Documented with evidence (in a footnote) just now here:
System-Wide Upgrade to Backports
However, once you manually install from backports, you will keep getting upgrades from backports, I think.
Why do you have the newer Tor version installed then? Because it was uploaded to the project Kicksecure repository for a few days.
(Whonix is based on Kicksecure.)
Citation required.
If you have the newer version, it is recommended to revert to Debian stable version as per:
Tor Version Downgrade