User ID Not Set in Events: Issues with User Grouping | Mixpanel Community

User ID Not Set in Events: Issues with User Grouping

How is it that the User ID isn't set in some events, even though I can see it in the 'View Users – Activity Feed'? It's incorrectly grouping some users under the cohort that only uses Device ID, even though those users actually have a User ID and when I breakdown by User ID it says not set and have triggered the event that should exclude them from that group.

2 comments

kapa.ai
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kapa.ai
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This situation can occur due to the way Mixpanel handles identity merging and event attribution, especially when using the Simplified ID Merge system.Key Points:Summary:
The discrepancy arises because the Activity Feed shows the merged user (reflecting the identity cluster), but event-level properties used for cohorting and breakdowns are not retroactively updated to include $user_id on events tracked before identification. This is expected behavior in Mixpanel’s identity system.If you need to exclude users from a cohort based on an event that should have a $user_id, ensure that the event is always sent after identification, or adjust your cohort logic to account for this identity merging behavior.For more details, see: - Identifying Users (Simplified) – Example User Flows - Identifying Users (Simplified) – FAQ
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