Train the tunnel to be high value. Use the rat as the reward after the tunnel is completed in practice. Raise the value when the dog is doing well to have the dog enter the tunnel with the rat at the opening.
Tunnels are darker in Senior but what I see mostly is inconsistency in the command asked upon the dog.
For example: Dog is asked to tunnel several times. Dog refuses to tunnel but goes off to hunt. Handler says ok find a rat.
This scenario may not be your exact issue, however, a command to perform a task should never become the dog’s decision to overlook and head off to self reward.
Have a plan and train to make the tunnel high value. This situation of not tunneling will not be corrected with trialing only.
Good luck, keep it fun and have a party when your dog tunnels!