While working on a ticket related to reports (log / loglive) I saw that there was an issue with the 
groups_get_members_join since we added visible groups in the query (MDL-79213).
I saw it when changing the content of get_group_filter.
 
This happens only when the course mode is set to VISIBLEGROUPS and you don't have access to hidden groups (mostly student). In this case the returned query will return all users who are in a group but not the users who are not set in a group.
 
This is a subtle bug but I managed to reproduce it on the participant page: * Create a course C1 with visible groupe enabled

• Create 2 groups A (visible) and B (hidden)
• Add 4 students
• Add student 1 and 2 to group A
• Add student 3 to group B
• Leave student 4 without group
• Login as student 1
• Go to the participant page
• Clear the filter
• Ensure you can see student 1, student 2 and student 4 (without group)
• Select 2 groups in the filter: no group and Group A
• Filter
• What is expected: we should see student 1, 2 and 4
• We only see student 1 and 2