Spark squeezing Images after saving

First of all: There is no Adobe Spark Board, so I hope it's ok to post it here.

I got a strange problem with Spark. I designed a posting with images from my library (quite large tif with alpha if that matters). Works well until I close the project and open it again, because this one image - and only this single asset, nothing else - is squeezed like hell. Any idea whats causing this?

before exiting the project:

after opening it again: