You're right. I did not know that sibuyanensis had the foliar nectar glands, but after reviewing photofinder, I see that they are pretty prominent!
I still think, though, that some of the white specs on the deadened leaves are sign of a pest of some kind, maybe meal bugs. Also some of the browner fleks around the base of the growth point look suspicious.
The strange growth of the leaves on the inermis does not resemble any sort of mite problem that I have experienced, and definitely looks like some kind of mineral, or fertilizing issue. Maybe a root problem of some kind. Have these been recently repotted correlating to the disfigurement?
Also as exo said, if the lighting is too powerful that wont help any either. If your using bulbs that give off hotter reddish light and they are too close, then these growth tips could definitely be having the life cooked out of them.