IPsec behind router without port forwarding

I have a IPsec N2N connection between 2 IPFire. One of them is behind a FritzBox. The Fritzbox is not working as exposed host and no port forwarding is set up, but the connection still works. Does anyone have an explanation for this?

Thanks in advance.

I have.
Not exposed host “calls” exposed host. Exposed hosts can receive and respond.
NAT keep answering to not exposed host.