Grid field sort is sorting one of the grid column autonomously independent of other columns

Good morning All,

we are using Gridfield to populate the task report data. In the task report we have one column which will bring back text values of tp!assignees or a constant value with some logic. When are using index function to populate the data in the grid. Everything works great until assignee's are null for a task.

      1. grid will sort the null values to the end of the list, irrespective of asc/desc order. Field sorting is not working properly for that Task Assignee column. All the null values are pushed to the end of the column independently. Rest of the columns are working fine.

      2. The rule is returning the values of the column in sorted order as shown at the top of the Image.

      3. Tried adding a default value in the index and the issue is that index is not recognizing this field as an null/empty value.

I appreciate the help and Thank you all in Advance.