The formula I use, is as follows:
=IF(G25="","",IF(YEAR(A25)<>YEAR(A26),YEAR(A26)&"-"&ISOWEEKNUM(activities[@[start_date_local]]),IFERROR(IF(activities[@[start_date_local]]="","",IF(C25<10,YEAR(activities[@[start_date_local]])&"-0"&C25,YEAR(activities[@[start_date_local]])&"-"&C25)),"")))