What i've found causes this is (as mentioned) the thickness of paint. but it also doesn't help if it is still soft or warm.
I found that if you either let it dry over night so that it isn't soft, or if its warm from being dried under heat, then blow some cool air onto it.
Other than that, just re-cut the line with the knife before unmasking.
If the paint is soft or warm, it will want to stretch the paint rather than it 'breaking' with the seam on the mask.
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