Recently one of my nepenthes leaves went yellow with a black center and killed of the leaf. This was unusual as the plant has been doing well and the leaf was somewhat new. I shrugged it off but noticed it's happening again to other leafs. I'm not sure what's happening or if it's fatal. Should I be worried? It's growing on my West windowsill 85F at day and 65F to 70F at night with humidity at 25% in the day and 60% to 70% at night. Any advice would be appreciated.