When I switched to the split view, the preview images re-appeared in the contact sheet. I even re-combined them, applied the stationary pad, and it worked. Of course, I don't want to have to perform these steps each time.
Also, (while I'm at it), when I look at DNG previews and contact sheets, the processing made in ACR is reflected in what I see. However, when I process an NEF file, no changes appear -- it still looks like the original unprocessed file. Is that just an ACR/NEF incompatibility, or is there something I can do in PM?
Here's what my prefs look like. I think they actually changed when I updated from 4.6 to 4.6.1:
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