Impossible to tell any specific cause from the image here. It would require a hes-res examination to see if this is an inking flaw or subsequent damage. The line is at a slight diagonal, which makes me suspect it is damage after purchase, or being at the end of the coil is damage from tape removal?
I took a good look and it looks like the line missed the color. It looks like it happen before the stamp design was added. Some red color is on the line specially the last stamp, also you can see that on the white section of the flag it did not effect the color. Please advice. Thank you Mara