technically this is not a bug.
however, if you feel it would suit your interests to have mIRC disconnect you as you have described, it would be a fairly simple thing to do via mIRC's scripting abilities.
I am not the best person to ask, so you might want to post over in the scripting forum, but here is the way I can see to do it off the top of my head:
1. set up a timer that is run when you first start mIRC or on first connecting that periodically sends a message to yourself with the $ctime variable. this does not have to be /msg or /notice, for example you could have it do a whois that would use the ctime, and personally I find this method is advantageous because unlike some commands the client sends to server, if I remember correctly whois will not reset the server's idle time for you, so it would not affect an away script that used the server's idle time for you.
2. set up an event to check for that message, and check the difference by: $ctime minus the ctime given in the message. if the difference is greater than what you want (ie, 10 seconds) then have mIRC disconnect and connect.
anyway, I hope this helps, because really this is not a bug with mIRC, it is something that can easily be done with it if that is what you want to do with it. all it takes is a little knowledge, so if you need help be sure to post it over in the scripting forums to get your help there.