Doing a little more work here, we found a handful (out of 4k) users who are actually reliably getting the activity summaries.
Comparing one of these users who does get summaries to one who does not doesn’t show any difference in their settings.
===== xxxxx (ID: 4149) =====
Active: true, Suspended: false, Silenced: false
Email confirmed: false
Digest enabled: true
Digest frequency: 10080 minutes
Last Seen: 2025-03-24 20:58:55 UTC
Last Emailed: 2025-10-16 17:07:53 UTC
Muted Categories:
User Stats Digest Attempted: 2025-10-16 17:07:53 UTC
Total Emails sent:16
===== xxxxxxx (ID: 42206) =====
Active: true, Suspended: false, Silenced: false
Email confirmed: false
Digest enabled: true
Digest frequency: 10080 minutes
Last Seen: 2025-09-14 15:52:54 UTC
Last Emailed: 2025-10-01 23:30:33 UTC
Muted Categories:
User Stats Digest Attempted: 2025-10-16 17:32:34 UTC
Total Emails Sent:2
Certainly there are other settings but I thought this was an interesting comparison either way.