Weird line on image with S_WarpCornerPin in AE

In AE I have here a still PNG image of a desktop with a couple of monitors on it & an MP4 AVC UHD corner pinned to the screen,
As the screen image & MP4 enlarge a line can be seen on the MP4, switching off S_WarpCornerPin makes it go away but I can’t work out what is causing it ?
It’s in the final render.
Mocha is just a mask & doesn’t seem to play a part in this weird line thing.

I tried a couple of other clips, added them to the composite & pasted the corner pin effect,
the line was on all, but I fixed it or at least made it go away by moving one of the outer corner pins :man_shrugging: