And what is expiry time? and how long ? 5 minutes? so if preferred uplink fails still all existing traffic will use it i.e. no connection to internet till "expiry" i.e. no internet connectivity for 5 minutes?
In case the link goes down, the failover would be immediate.
But a soft failure occurs when the link is still up, but there is no connectivity over it.
All flows going over the Non-Preferred link will continue to be bound to the Non-Preferred link until 300 seconds pass without seeing any relevant frame or if TCP ended the session related to the flow on the Non-Preferred link.
In this scenario, there is a chance that you'll have an active flow that will remain active on the Non-Preferred link after hours since the recovery of the preferred link.