MariaDB server crashes when replicating 10.6->10.5

Replication from a server to another with a smaller major version numbers is not supported.
However, if one is trying to do that the slave should crash.
In case of any replication protocol incompatibility or not supported sql queries, the slave should give an error and stop replication gracefully.

In theory, as long as there is no new replication features used that has caused changes in the protocol and all queries are also supported in the older version, the replication 'should probably work', but no guarantees.