synaptic does not abide the “Default-Release:” apt preferences setting.
Per the default synaptic configuration, it will choose “highest version” as the installation candidate.
To change the default synaptic behavior, you can edit synaptic.conf or visit the “Distribution” dialogbox
The behavior of synaptic was the first thing I looked into.
Although the settings are as skidoo mentions here(prefer the highest version), no package from the backports is shown as an update.
So,yes synaptic acts as smart as apt in this case.
If you want to update a package from backports with synaptic, you have to checkmark “prefer version from buster-backports”