I'd call this a bug in mirc. I connected to a server running UnrealIRCD and mode f was correctly listed in the 2nd (type B) field of the CHANMODES token in raw 005. Yet when I created a channel and set mode +f [10j]:15, mirc didn't add the parameter ([10j]:15) in the titlebar or in $chan().mode. However, when I set +k somekey (k being of the exact same type as f) mirc updated the titlebar and $chan().mode with the correct key information. So it seems mode f isn't interpreted correctly as a type B mode, even though mirc has been provided with all the necessary information for this. The same happens with the type C mode j on UnrealIRCD: titlebar/$chan().mode aren't updated correctly by a mode j change, however they are updated correctly by a mode l change (same type of mode).
/.timerQ 1 0 echo /.timerQ 1 0 $timer(Q).com