When the ICU for Unicode is updated or changes 4D will index related fields. The fields that will get reindexed are text fields and object fields. If a database if opened with newer version of 4D a reindex can occur due to this.
For example, 4Dv20 has an updated ICU version compared to 4Dv19. Opening a database from v19 to v20 can cause a reindex if the database contains text or object fields.