The missing-menu automation looks back as far as it needs to — up to about a month — to find the most recent day that actually has a menu on that channel, and copies from the first one it finds. If a channel hasn't had a menu built in a while, it will copy from however far back it has to go.

If you don't want stale menus carried forward indefinitely, either build a fresh menu manually, or turn off Select Previous Menu if Meals are not selected for Delivery Date on Cutoff Day (Client Config, tab Theme: Kitchen) and rely on the missing-menu email instead.