Weird: Some Render Cache clips not persisting on restart

This specific case is strange.Some clips persist, others, the exact same grade and compound status, do not.

Of the 6 clips in v1 in this pix https://www.dropbox.com/s/304ij4qizudgts3/Capture.JPG?dl=0 clips 1 and 2 cache renders do not persist while 3,4,and 5 do. Yet 1 and 3 (both compound) have the same grade as do 2 and 5 (not compound). 6 doesn't persist either but a it's more complicated shot and the cache render takes a long time so it's turned off.

So it doesn't seem a matter of compound vs not compound or grade vs grade but something entirely different.

Here's the project file https://www.dropbox.com/s/ij7lvhgcfkerlmg/Opening.drp?dl=0. The sequence begins at 00:05:49:00