In general, CAT allows a user to approve any or all suggested tags for an image, and adds all approved tags as Depicts statements. However, for at least one image, approving any two or more suggested tags causes MediaWiki to detect an edit conflict when attempting to add Depicts statements. The problem is reproducible on both Commons and Test Commons for this specific image, and happens regardless of the logged-in user.
https://test-commons.wikimedia.org/wiki/File:Malibu-superbloom-redux.jpg (deleted for testing)
After adding some logging, the error Status coming back from the revision save attempt is coming back with a value of EditEntity::SAVE_ERROR (8), which appears to indicate that a StorageException was thrown while calling $this->entityStore->saveEntity() on L722 of MediawikiEditEntity.php. The error Status also has the message edit-conflict, which indicates that the StorageException in question is the one at L357 of WikiPageEntityStore.php. But there is no apparent reason that the edit conflict check there should consistently fail for this specific file.