Odd. I have not been able to recreate that myself outside the Lilou page, which insists on a window size much bigger than my game. On itch, I have found no problems. After the jam, I will see if I can add a version with a larger image for the start screen.
Thanks for bringing this to my attention!
Edit: Are you playing the Windows version? Just checked it on my computer and if the window is maximized, this issue does appear.