The group by behaviour with v2.8 seems to add new columns in the flow to the 'group' table without explicit action by the user. This can cause problems: I was wondering if this can be disabled (eg. enforce inclusion button) or reverted back to previous versions?
I assume that you mean the Group Loop Start node with the group loop nodes. This node allows you to specify in the dialog if new columns should be placed in the include or exclude list. As in your case if new columns should be placed in the exclude list select the Enforce inclusion option at the bottom of the include list. If new columns should be appended to the include list select the Enforce exclusion option at the bottom of the exclude list.