Hey Mark,
Welcome to the ACA Forums. While I will let ACA post the official response, historically, they only update and reprint (not just reprint) a map set only when 1) an update appears to be needed, i.e. the old route has become too congested with cars, lots of "addendum" have been added, etc. AND 2) they are running low on maps. They do not automatically update a map each printing cycle if no update is needed; they just reprint the maps.
It takes quite a while to update and reprint a map believe it or not. For instance, they are updating parts of BR66 now that will be available I "think" later this year or next year. To update and print a route, ACA has to basically create a "new" route for any effected map panels and then possibly reroute adjoining panels. Then they have to redraw the effected panels and the accompanying text which may radically change the overall (the entire folded printed map sheet) map layout to keep it all a printable size. This is in addition to having to research the new panel's services. Once all that is done, they have to do the actual proofing and printing process.
All that said, it would be nice if the website had a "map status" page which showed when the next update, printing, etc. is being done. However, to be fair, such status should be highly suspect since lots of issues may pop up that could change the timing.
Tailwinds, John