MySQL command line tool can ignore "--port" argument without giving warning

I have a box running MariaDB at the usual port, 3306.
It also has a MariaDB running inside a container. The container exports to port 3310.
Assuming $IP is the IP address of the host, then when I say (on the box):
mysql -h $IP --port=3310

then I get connected to the containerized MariaDB, as expected. But if I leave off the host specifier and just say:

mysql --port=3310

then I get connected to the non-containerized MariaDB at the default port, 3306. There is no warning or error message that this is happening, even if I specify "-v -v -v". This is a bug. It should either not do that, or else it should issue a warning, or else the documentation should specify this behavior.