Most Miscanthus cultivars are good to zone 5 and colder, so I doubt it's winter kill. I also question if the fall trim did them any harm, unless possibly you trimmed them before they had gone dormant. Even that shouldn't kill them.

Mine were later than normal coming in this year, also. We had the coldest winter in 10 years, and the coldest weather had no snow cover.

I was thinking along those same lines, so I'm not sure what happened. It does seem to be only the ones I trimmed in the fall, maybe it was too early a trim for them after all. Some of them really seem to be struggling with only a few sprouts, the one I left uncut looks absolutely fine.... But the arundo I cut back last fall at the same time seems unphased, so go figure.
Maybe next fall I'll try the same treatment on only half and see what happens, but then again slightly different timing, different winter, who knows what that will prove?
Thanks for your thoughts.