Offices will reappear given time, providing a suitable candidate is in the right place, but we included a specific condition called FactionwideAncillaryExists to prevent more than one instance of a particular person/thing existing per faction.

Take the magister scrini, the head of the Imperial secretariat

FactionType empire_east
and EndedInSettlement
and RemainingMPPercentage = 100
and SettlementBuildingExists = imperial_palace
and IsGeneral
and not IsFactionHeir
and not IsFactionLeader
and Attribute Management >= 1
and Attribute Influence >= 2
and not FactionwideAncillaryExists office_magister_scrinii_east
and Trait Loyal >= 5

This office will re-appear at some point in the future if the holder dies, assuming that all the conditions are met. There is, of course, an equivalent office_magister_scrinii_west for the WRE, and this is a different office even if it has the same on-screen title.

Hope that explanation helps.