WFS: DescribeFeatureType query with typenames?

I am struggling with the DescribeFeatureType query with WFS KVP encoding.

I have this WFS: https://inspire-wfs.maanmittauslaitos.fi/inspire-wfs/au?request=GetCapabilities&service=wfs&version=2.0.0 and I can see in the GetCapabilities that there is a FeatureType called au:AdministrativeUnit.

I could of course use DescribeFeatureType to give me the description of all Feature Types of the service, but I would like to use &typeName=XXXX in the kvp to choose only this FeatureType au:AdministrativeUnit

So I basically do the following request: https://inspire-wfs.maanmittauslaitos.fi/inspire-wfs/au?&request=DescribeFeatureType&service=WFS&version=2.0.0&typeName=au:AdministrativeUnit

But the result is not filtering at all to give me only the Feature Type with the name au:AdministrativeUnit.

I saw that in the OGC specifications, there is sometimes written typename or typenames. I tried both of them with changing the version and I don´t get it!

Am I missing something important here?

With an other WFS, I can do the query with filtering typenames without issue!