My Nep lids wither prematurely, please help.

[Durban]

Hi everyone, i have a problem which i hope some experienced people here can help me out with.

The lids of two of my Neps always brown prematurely, after about a month. They do this in no discernable pattern, sometimes it will be the newest pitcher, leaving the older ones fine. Only my N. Albomarginata and N. Veitchii are affected. Also, some of my Albo's leaves are a bit yellow, not the older ones either, just random. Both plants are growing very fast and putting out large healthy pitchers. I have checked for pests, there are absolutely none, i even checked under the lids as i know thrips like it there sometimes! Here are some of my cultivation details:

Light: 14 hours artificial
Humidity: Day: 70%-90% Night: 80%-95% NEVER drops below 70%.
Temps: Day: 29c-35c Night: 24c-28c
Water: RO water, 0 ppm, i water from top only.

What could this be?, it can't be low humidity as it never drops below 70% even when temps are 35c. Maybe it is the occasional rise in temperature to 35c for a few hours? I have reduced watering as i though it may also be that, time will now tell. Any opinions would be well appreciated.