I haven’t yet come across a not seekable PNG. There might be something unique about the ones you’re using. Please upload one of the images so that someone might look at it and figure out why it’s not working.
What is your operating system, Shotcut version, and where did you get Shotcut? Your build or installation seems broken. This is not reproducible by me.
The PNG you provided works fine for me. Maybe you uploaded one of the PNG that works for you instead of one that does not. One that does not is useful here.