Function elements like:

<Function Name="isroot" IsBound="true">
<Annotation Term="Core.Description"
String="Returns true, if and only if the value of the node property of the specified hierarchy is the root of the hierarchy" />
<Parameter Name="Entity" Type="Edm.EntityType" Nullable="false" />
<Parameter Name="Hierarchy" Type="Edm.String" Nullable="false" />
<ReturnType Type="Edm.Boolean" />
</Function>

are missing Nullable="false" in the ReturnType elements.

Clearly it wasn't intended for these functions to return nullable Boolean values, so this should be fixed.