There are only 5 items in the list.
just tested it on my local environment and having the exact same problem you describe. I can't even see any activity when trying to expand the group. Not the browsers dev tool nor fiddler show any activity.
Seems like this control is just broken when showing a view with grouping
I can confirm just the same behaviour on NF 126.96.36.199.
what form release are you on?
there are reported some bugfixes for list view control in recent releases.
I you are on current release I would suggest to open a bug ticket with nintex
I tested on 188.8.131.52, issue still exists but it's not the most recent version
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