connection keep alive still closing and idle detected

Hello,

On a read/write split service, I have this error log:
Lost connection to the master server, closing session. Lost connection to master server while connection was idle. Connection has been idle for 399.6 seconds. Error caused by: #HY000: Lost connection to backend server

on our mariadb server, the wait_timeout is set at 400 , if I change that value, the maxscale log error changes too.

in the maxscale.conf, I have: connection_keepalive=40 under the service section.

why do I have this lost connection to backend server, the connection_keepalive is supposed to keep it alive even if idle ?

thanks in advance