Yes, it's probably either what you suggested or some sort of server flood protection. I have the same thing, and I've had it since 5.91 (the first mIRC I used).
I don't *think* this is a bug. In fact, it's probably a good thing that it happens - if you got query flooded by 50+ bots, and were forced to /whois all of them, you'd either disconnect or get extremely lagged.
Regards,