I too am having a similar issue with the Group Filter widget. I'm using Portal 10.6 and the Reset button doesn't reset and un-apply the filter. I don't have the 'Persist after widget is closed' setting checked. It doesn't reset when I close the widget either as the documentation suggests that it should.
These are the results of an applied group filter displaying a number of records returned for the selected filter value
And here is the same attribute table after the widget has been both 'Reset' and Closed which should have reset the filter completely. BTW, there are 996 total rows in this layer when it's unfiltered.
Is there a bug with the 'reset' when the application is closed?