Whenever I use “Crop: Circle” in conjunction with “Size and Position”, the circular form stays on screen but when I move the “Size and Position” filter around it does this.
I reproduce this, but it is a side effect of a performance optimization that does not appear in the exported result in my test. You will notice that if you change the Crop: Circle > Radius the preview glitch goes away, and the smearing part is updated with the Color. Change the radius back to what you need. That is the workaround for now. Removing the optimization is having some unintended result to resolve this now. I bookmarked this to try to fix it again later.