Just struggled with this myself today.

8.0.33 apt repository (current as of Jan 2025) for Ubuntu 24.04 has mySQL Router 8.0.x, which doesn't recognise the r/w primary server on later versions like 8.4.x (can see in the log all three servers are ro, and then it shuts down the r/w ports because there's no r/w server).

Manually install the version you need like 8.4.x

wget https://dev.mysql.com/get/Downloads/MySQL-Router/mysql-router-community_8.4.4-1ubuntu24.04_amd64.deb
sudo dpkg -i mysql-router-community_8.4.4-1ubuntu24.04_amd64.deb

and re-bootstrap it and it will pick it up and work.

The mysqlsh version that comes with that repository is also bad and doesn't have the --js function, so recommend manual install of that as well.

I assume this will be fixed next the apt repository is updated.