In short: I live in Europe and DAX/WEEKNUM does not give the same results als other Microsoft products like Outlook and the DAX/WEEKNUM logic is not according to the ISO 8601 calendar standard. Please change this.
By default, the WEEKNUM function uses a calendar convention in which the week containing January 1 is considered to be the first week of the year. However, the ISO 8601 calendar standard, widely used in Europe, defines the first week as the one with the majority of days (four or more) falling in the new year. This means that for years in which there are three days or less in the first week of January, the WEEKNUM function returns week numbers that are different from the ISO 8601 definition.
So now with this behavior Power BI will differ from other Microsoft applications and also the European standard.
The only option I now see is making a custom date table but that has a lot of impact if I have to implement that in all our reports.
My suggestion is changing the DAX/WEEKNUM logic to match the logic in other Microsoft products like Outlook and thus the European standard.