Excel gantt chart help

Hi,

I am trying to make an "milestone diamond" to my Excel Gantt Chart. The equation works for some dates, but it doesn't work always when I change the date. For example: Now (red circle) the date is 1. January 2024, and the diamonds are there. But, when I change the date to 1. February 2024, the diamonds disappear. I want that the diamonds are there no matter what the date is. What would be the correct formula for this purpose?

Now it is: =IF(AK$6=$G9-WEEKDAY($G9,2)+1,"u","")