Replicator doesn't keep a mask

I'm following through a tutorial to build a video wall. Cool tutorial, great effect, but one glitch I cannot figure out.

I start with a drop zone, and I put a circle image mask on it, to create rounded corners on the drop zone.

That drop zone gets replicated, and I arrange it into a 4 X 4 grid. Only the first 2 images have the circle mask on it, the rest are unmasked with square corners.

How can that be? Why would a mask on the replicator work twice, and then not on the rest of the replicated cells? Is this a glitch? Or is there something I can troubleshoot in a setting somewhere?