No, the system is designed intentionally not to fail back automatically but is optimized to ensure maximum uptime.
If the primary failed (for whatever reason) the admin has to take a look and switch over manually if everything is ok. Otherwise you could run into situations where the primary comes up for a short time (maybe a latent hardware issue) causing a series of reboot/failover/reboot/failover in a loop.
If the traffic stays on the secondary there is no interruptions for the user and besides the naming "secondary" it doesn't matter which firewall is active. If the secondary would fail, and the primary is up the primary would automatically take over anyway.