If chanmode +f is set, the channel limit takes the first number of +f x:x and shows up as the channel limit.
So if you set /mode #chan +f 5:10 and then /mode #chan +l 15, mirc will show the channel limit as 5 when you rejoin that channel.
Here is the information provided by the IRCD Software:
--- TOPICLEN=307 KICKLEN=255 MAXTARGETS=20 AWAYLEN=200 CHANMODES=Iqageb,k,LfJjl,SzrRMOTNQVCKGcpsmnti FNC NETWORK=StaticBox MAXPARA=32 EXCEPTS=e INVEX=I OVERRIDE REMOVE SILENCE=999 :are supported by this server
i dont see why it should ignore the +f, it is published in the 005 numeric.
If you need any more information let me know.
We use the InspIRCD software