For me, it happens with every sequence, no matter scaling, frame rate or something else…
At first, I also thought it was the Sharpen effect but I deleted and the problem is still here.
It happens each time I use a Shape Mask on an effect situated at the beginning of the list. Like you said, if I reorder my effects and put the one with the shape mask at the end, no problem!
But now, I really need my effect with Mask at the beginning, before applying my LUT… I don’t understand why this problem exists and what I can do…