When adding new fields to the search indices or making minor changes to how the content of a field is generated the only way currently to ensure that change has been applied to everything is to run a long (week+) maintenance script to rebuild the indices from the database. This is generally avoided, it's not been run on the largest wikis in years, but means we have documents that don't have all the fields they should.
Resolve this by constantly reindexing documents. This must provide a guarantee on the oldest possible last indexed date. It would be nice to have finer grained information about when documents were indexed and proportions after some deployment date, but only a guarantee on the oldest possible indexed document is required.