funny thing I was seeing this issue for long time.
Nextcloud and Collabora running on the same Docker host where talking to each other using public IP despite the fact working splitbraindns was in place and internal client, internet router and Docker host itself resolved the dns records to the local IP of the Docker host. I didn’t find the root cause - likely it it lies somewhere in the depth of routing and iptables used when you setup Docker containers.
long story short - I played a little with internal Docker DNS and found a good workaround - just add all the public DNS records as alias to reverseproxy container. for details look here