I share your concern and have a similar thread here:
I’d recommend you change the standby state to disconnected to limit your usage until a better solution is available.
If applicable you can also do what I am doing which is to turn it off during non-peak time. Keep in mind the current challenge is that it will be completely down regardless of the health of the other wan;s
On BR series it’s called Standby state that you want to change.
On Balance series it’s called “Connection Priority”
Change from Always-on (Priority 1) to Backup
You may also want to consider checking “Independent from Backup WANs”