Grouping by functionality in List v3 produces incorrect results.
Steps to Reproduce
1. Navigate to task.list.
2. Group by "Assigned to".
3. Click on a few of the users underneath "Show All" in the Groups pane.
Although the list should show only the records where the Assigned to field is set to the user you've selected, it shows the records where the Assigned to field is set to a user other than the one you've selected.
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Related Problem: PRB1260888