TF2 Server resets mp_idlemaxtime and instantly idle-kicks players

[Bruno02468]

I've recently created a TF2 server, and my server.cfg sets mp_idlemaxtime to 10.
After starting the server, the cvar's value is still 10.
But, as soon as any player joins, they're kicked for being idle.
If I check the cvar's value, it changed to 0 (probably the reason for kicking) for no reason!
Does anybody have any idea what may be causing this and how to fix it? No one can join my server because of this

Thanks!