I know this post is quitte an old one. But i'm having the same problems as well.

I have defined an structure were i return the weeknumbers, the year and the startdate of that week. However when i'm going from week 53 in 2020 to week 1 in 2021 i don't get the returned date as desired.

With this set of input parameters 

Where Year = 2021

Weeknumber.out = 1

I get 04-01-2021 returned as the startdate which is correct !

However when i set the input parameters as:

Where Year = 2020

Weeknumber.out = 53

The nulldate returns

Anybody got a suggestion for this issue ?

It seems like the weekrule you're able to set as an input parameter has a strange behaviour between years. It would be possible to just set the values in an static entity to get the desired output for each year. But that doesn't seem like it supposed to work like that.