Why does my vanilla cream filling soften my copycat Oreo sandwich biscuits?

My filling is low sugar and I use oil, not butter or shortening.

Edit: Filling ingredients 500 grams cashew nuts 333 grams milk powder 175 grams confectioner's sugar about 120 grams water

I might have added some oil because it was too thick. Water was added was for the same reason.

Edit: Too thick to spread. But never mind. I finally found a commercial recipe.