turn on all these flags and see if the debugger points you to an issue in your code. https://useyourloaf.com/blog/debugging-core-data/

This sounds related to the current issue that support for DiffableDataSource in FetchedResultsController doesn't handle updates, only inserts, deletes, and moves. FetchedResultsController is the backbone of @FetchRequest in SwiftUI. See https://www.avanderlee.com/swift/diffable-data-sources-core-data/ for a good discussion of the issue.

Hmm

Do you have a Searchable index on favoriteColors?