I can't help, but I have some P. vivax that I planted last year, and it died back to the ground like your nuda -- in my case I'm assuming just because it's an immature stand, but maybe it was the hard winter -- and the new culms are very floppy, too.

The stand is.very healthy. Plenty of new shoots and looking good. I cut out a huge number of older culms early. I am just curious about the ones that flop over. I get plenty most years that are killed or misshapen by late frosts but this seems to be a dietary problem.