Now Microsoft Monthly Patch usually require Service stack update need match spec date.
and ZPM seem usually has such rule.
so like my customer usually deliver microsoft patch every season...so some workstation maybe could not see the latest patch list..because his installed SSU not matched , and ZPM also not provide servicing stack update in list (it should be hidden)
Customer must manual download Servicing stack update (SSU) to install , then run zac ps ...the latest patch list return well.
ZPM should provide all versions of Servicing stack update let managed device could install it to match monthly patch requirement.