SQL Router Not Applying Round Rubin
Every connection has a -h 127.0.0.1 or as you have a --protocol=TCP" on local computers to use round-Robin.

The problem is on linux computers mysql tries to open a socket connection, where you can' use Roud-Robin.

The same is true for Windows computers, but there mysql uses named pipes

so you have to force the connection to use tcp/ip, that works with an ipadress in -h or like you used forcing the protocol