openpyxl: (column) grouping - outline level not working & retaining only last one

I am trying to group columns at multiple levels and in this process only the last outline level is retained, though the outline number area in spreadhseet shows all outlines. I referred to this past post and it worked initially and it stopped working for the below code:

Here is my code:

from openpyxl.utils.cell import get_column_letter

def group_xlsx(output_worksheet):

    start_row=1 #start row
    start_column=2 #start column
    total_columns=48 #total columns
    columns_number=3 #columns in each group
    
    #outer group
    first_column=get_column_letter(start_column)
    last_column=get_column_letter(total_columns+2)
    output_worksheet.column_dimensions.group(first_column,last_column,hidden=True,outline_level=1)
    
    print('Outer group columns are {} & {}'.format(first_column,last_column))

    #inner group
    for index in range(start_column,total_columns+1,columns_number+1):
        first_column=get_column_letter(index)
        last_column=get_column_letter(index+2)
        output_worksheet.column_dimensions.group(first_column,last_column,hidden=True,outline_level=2)

    print('Last inner group columns are {} & {}'.format(first_column,last_column))

    return

This is how it looks in spreadheet

Not sure where it is going wrong. Any suggestions are greatly appreciated.

With the way it is formatted in Excel, you'd want to do something like the following to achieve what I believe you are attempting;

from openpyxl.utils.cell import get_column_letter
import openpyxl


def group_xlsx(output_worksheet):
    
    # outline_level 1
    for i in range(5, 50, 4):
        output_worksheet.column_dimensions.group(get_column_letter(i), hidden=True, outline_level=1)
        print(f"Setting group for separation column: {get_column_letter(i)}")

    # outline_level 2
    for j in range(2, 50, 4):
        output_worksheet.column_dimensions.group(get_column_letter(j), get_column_letter(j+2), hidden=True, outline_level=2)
        print(f"Setting group for {get_column_letter(j)}:{get_column_letter(j+2)}")

    return

filename = 'grouping.xlsx'

# Create Excel
wb = openpyxl.Workbook()
ws = wb.active

# Call function
group_xlsx(ws)

# Save workbook
wb.save(filename)

The Sheet should look like the following if I read what your trying to do in your code correctly.
_{Obviously with the groups expanded}