I don't know Parnassia foliosa var. foliosa but am very familiar with the European Grass of Parnassus which I have successfully transplanted into my mountain garden.
In the wild it grows both very wet but never in stagnant water, or rather dry but always in calcareous soil. It is often associated with plants like Saxifraga aizoides and Gentianella campestris, and also Dryas octopetala.