In US week numbering i believe that is actually valid, because week 53 can also be week 1 of the next year. Given that in the screenshot the weeks start on sunday, i assume US numbering applies. They probably have a setting to switch to ISO week numbering where it would indeed be week 1.
I think they coded it for European style calendar format "Monday" = first day of week, as SAP is a german company. I wrote a datepicker in JavaScript and it had similar bugs in one of the calendar formats too.