Film on top of my almost vegan potato / onion / parsley soup
Since you mention the film develops after cooling, it is likely to be a very thin layer of fat.

There are some sources of fat in your recipe: the butter you sweat the onions and garlic in, and the bouillon cube. Both of these are very minor sources of fat, but it's still present. A quick stir after warming should incorporate it back in, or you can instead skim it off when the soup is completely cooled.