Same here... I did a quick test by spinning up a Docker container on my machine that logs all incoming requests and configured the Flic Hub to send the requests to that container.
The results are: Flic hub has no problems with sending requests to IP addresses, but requests sent to hostnames or FQDNs won't arrive at their destination (in this case the Docker container).
For me this workaround of using IP addresses is fine, because I will trigger scripts of my Home Assistant in my local network, so I can use the according IP address.
But this should definitely be fixed for domains / hostnames!