Report Sort Order Seems Broken

I have a list of products by t-shirt size, (S,M,L), i wanted the specific line items to be grouped and sorted by t-shirt size, but it only sorts them as L,M,S in alpha order.

So i created a helper column called sort order which assigns 1=S, 2=M and 3=L.
When i sort the report by sort order it works, but as soon as i group it by t-shirt size the sort order goes back to alpha by t-shirt size eventhough i specifically have the sort to be by the column "sort order"

Why is this happening? Is this a bug?

Answers