It appears that the "on OWNER" event triggers when a +q entry is added to the channel list on FreeNode. However, on FreeNode +q means "quiet", which is more like a ban. I believe that mIRC should be checking against the 005 PREFIX= token to see if +q is actually a channel-member prefix, and not trigger "on OWNER" (etc) if it's not.
As a sidenote, it probably wouldn't hurt if "on OWNER/DEOWNER" and "isowner" were added to the help file. I don't know how widely used the owner feature is across IRC networks, but people do ask about it every once in a while, and it looks like it's only in
versions.txt right now.
Edit: credit where credit is due, this bug report is an (indirect) result of an observation by KindOne.
