The order of the options in a Single Select field are defaulted to alphabetical when accessed via a Lookup field. This means that grouping by the lookup field ignores your assigned order, which is very frustrating. If the options are ordered a certain way, it is a disservice to violate that. Please preserve the order of the single select field through lookups.
If for example I have a Single Select field with options: “Priority” “Casual” “Delisted”
When I access that field via Lookup and Group my view on it, my records will come grouped in the following order: “Casual” “Delisted” “Priority”