I had problems using MaxScale SchemaRouter in conjunction with applications like Matomo (Piwik), phpMyAdmin and Directus. The setup consisted of two MariaDB servers with the application databases residing on either one of the servers.
In case of Matomo there were errors like 'SQLSTATE[3D000]: Invalid catalog name: 1046 No database selected - in plugin Actions' in the logs. The Matomo docs state the following 'MySQL' requirements: 'Matomo won't work with a MySQL load balancer such as ProxySQL unless you can ensure that all queries within a given established connection are always sent to the same database server'.
I guess the reason for the other applications to fail has a similar cause. It seems that MaxScale isn't able to properly route the queries to the right server in such a setup ?because it tries to do ShardRouting instead of SchemaRouting (just a hunch)?