multi-track compositing doesn't render correctly

I have 5 video tracks and have composited them so that the first 4 tracks are at 45% in each corner and the 5th is background.

The project monitor plays back exactly as required.

When rendered (Youtube, 1 pass 1280 x 720 px) , the very first frame is correct, but after that the first three corners all have track one, and the fourth corner has track four.

I can't see what I've done wrong. Any suggestions?