I’ve noticed this as well (along with a number of the other System statuses).
Root cause could be that the system status for expired maintenance has two conditions loaded into the “AND” side (but I can’t tell what) which is likely creating some sort of paradox for it.
I was able to create my own custom status for expired maint (select Internal Control Maintenance - Expired → is in the active status) and get that Status back (should be a short term fix). I suspect for any other missing Status, you can do a short term fix by recreating it as a custom status.
Looking at other “expired” rules (like audits), it appears two conditions are used → 1. Related Audit is Current AND 2. Related audit is expired in order to generate the Status. Could be a consideration when making the custom one…