Well, the title says it all. Columns of type Set are handled correctly in aggregations (namely 'Count' vs. 'Unique Count'), but when used as grouping columns, sets with the same content may be considered to be different.
Attached: Minimal example to demonstrate
thanks for reporting the bug. I can confirm that the groupby node has a probem with set cells if the elements of euqal sets appear in a different order. We will have a look into the issue and fix it as soon as possible.