When the contract has been finalized by the agreement between the third party and the agent (on behalf of the principal) it’s the principal that can enforce it and the agent’s involvement is complete
I don’t know how else to explain it – parallels with hands and bodies have been occupying my thoughts to see if I could use those as an illustration … but there’s nothing to stop the hand getting involved again when the third party breaches the contract
You’ll just have to accept that, once signed by the agent on behalf of the principal, the agent’s involvement is ended