It's not the query string... it is the root name of the app.
readmsg.asp is used in profile.asp, board.asp, search results, etc. If the user browsed posts with read_msg_ig.asp they are most likely not going to be in the browser cache as read_msg.asp, Unless they also happened to browse them with read_msg.asp previously, which is quite unlikely.
Toggling the read_msg(_ig).asp URL in other apps depending on the user's ignore state won't fix it since they could have been in either ignore state when the posts were originally browsed.