The port number is not (and was never) a fixed quantity, and Roon API extensions were never intended to hard-code it.
It has always been assigned out of a range, where we look for the first available port. 9100 was the first port in that range, so on many machines, Roon would use 9100, but there are many people who have been using 9101 or 9102, and quite a few users who ping-pong around based on the order that various software starts up.
We moved the port range because we became aware of several pieces of software that are inflexibly tied to 9100, and due to implementation errors in that software, they would sometimes result in conflicts.
For Roon API extensions, the correct approach is to use the discovery protocol to discover the Roon Core and the correct port, as exemplified in our SDKs, for example GitHub - RoonLabs/node-roon-api: Javascript Roon API .