On a No Groups forum, when I navigate to the User posts page (via 'View posts
by user' button) I am able to select a group of which I am a member, but it
results in a permissions error.

As a Tutor

1. On a group forum

After I select 'View posts by user' I see both my groups in the drop down list.
When I change between groups, the User table changes to reflect the different
users in the groups. (Expected functionality).

2. On a no group forum

After I select 'View posts by user' I see both my groups in the drop down list

When I select the alternative group and select 'Set' I am getting a Permissions error message:

'Sorry, but you do not currently have permissions to do that (Access all
groups)'

the course has a default grouping where the tutor is not present in one of the groups.