Apparently there is a problem in CloudKit with re-indexing records after the index has been altered. I can filter new records (added after the queryable field was added to the index) successfully, but "old" records continue to be ignored. I don't see any option in the CloudKit dashboard to force re-indexing.
The price you pay for giving away control of your backend.
Topic:
App & System Services
SubTopic:
iCloud & Data
Tags: