This sounds like a BuddyPress AJAX response conflict rather than a failed term deletion. Since the term is actually deleted and WP-CLI works correctly, the issue is likely caused by unexpected output, a slow hook, or malformed JSON during the admin AJAX response after wp_delete_term() runs. The fact that it starts happening with a large number of taxonomy terms strongly suggests a scalability/performance issue, possibly related to BuddyPress activity tracking, notifications, or taxonomy-related hooks firing during create/delete actions. Check for BuddyPress hooks attached to created_term, delete_term, edited_term, or shutdown, and test by disabling BuddyPress components individually (Activity, Notifications, etc.). Also inspect the browser Network tab for failed AJAX responses or invalid JSON output.
