My attempt

src/components/Grid/useDataGridSource.tsx

@@ -385,13 +385,16 @@ export const useDataGridSource = <
   const onSortModelChange: DataGridProps['onSortModelChange'] & {} =
     useMemoizedFn((next) => {
       let sortModel: ViewState['sortModel'];
-      const initialField = initialSort[0].field;
-
-      // Covers the case where the initial field is the same as the current sort field and initial sort is 'desc'
-      if (initialField === view.sortModel[0].field && next.length === 0) {
-        sortModel = [{ field: initialField, sort: 'asc' }];
+      if (next.length > 0) {
+        sortModel = [next[0]!];
       } else {
-        sortModel = next.length === 0 ? initialSort : [next[0]!];
+        // If "un-sorting" revert to initial sort
+        sortModel = initialSort;
+        // If the prev sort field _is_ the initial sort field, be sure to flip order
+        const prev = view.sortModel[0];
+        if (prev.field === initialSort[0].field) {
+          sortModel[0].sort = prev.sort === 'asc' ? 'desc' : 'asc';
+        }
       }
 
       setView((prev) => ({