I just had this same issue with my ampullaria but I believe my problem to be heat related. During the fires and Santa Ana winds that we had here recently, the temperature in my Lowland tank skyrocketed to around 100+ degrees (while I was not around). My humidity has always been consistently high, and so I know it was not humidity related, but definitely caused by stress (heat). All the lids on my Amp closed up, and the plant got rugged looking. Anywho Nepenthes Mira seems to be a fairly easy Nep to grow, at least in my experience. Good luck and keep us posted =).