I had similar problems with my Fritz 7490.
In my findings, it's related not to the port forwarding itself, but the Fritz notion and behaviour of the destination machine ("to computer" field in your screenshot).
Example: If the same machine (one MAC address) boots up different operating systems/hostnames, Fritz gets confused, and that affects port forwarding. Also if you use the "specific IP address" the port forwarding configuration, but then later change/add a machine config with the same IP address, the port forwarding changes to point to that machine "name" instead of the IP.
In summary: try to clean up the machine/MAC list in Fritz (in "Home Network > Home Network Overview > Network Connections"), making sure there's no conflicts, and re-create the forwarding rules. That should fix it.