We have the same problem. Is there a way to solve or go around it?
A reply would be nice.
This could happen for WeekStarts=Sunday. The week number calculation is based on ISO 8601 which uses Monday as the first day of week. The supplied Sunday belongs to a previous week according to the standard.
It's fixed now in build 6.6.2336:
It finds the Monday of the current week and passes it to Week.WeekNrISO8601() method.
The workaround would be to use custom HTML for the header using BeforeTimeHeaderRender event handler (for e.IsColGroup == true).