Dropbear IdleTimeout

I want my openwrt 22.03.2 dropbear to drop incoming ssh connections in case of inactivity, so I set IdleTimeout of dropbear to 600.
On the client side I use

ssh -o ServerAliveInterval=60

to send null packets to keep the connection alive. However the connection is dropped anyway:

Mon Nov  7 17:42:38 2022 authpriv.info dropbear[4940]: Exit (username) from <x.x.x.x:12850>: Idle timeout

What am I doing wrong?