As I understand it that used to be the case, no way to kill the states that selectively, and that's why it behaves as it does. However I believe current versions of pf can in fact do that so it may be possible to do exactly that now.
I have this issue myself. My backup WAN is data limited so I can't have connections using it continually after my main WAN glitches. Most traffic will fail back naturally as TCP connections close it's only really persistent UDP traffic that gets 'stuck', so VoIP and VPNs. My own WAN is good enough that I just don't failover that traffic but I understand that for many those are the most important connections.
That said if it did just kill everything on the backup WAN when the main WAN comes bacl up it would needlessly interrupt that traffic. A cron-job that kills traffic on the backup WAN that runs at some off-peak time might be better.
Steve