I agree leave it be for now and keep the humidity up, it might be just some simple stress.
Funny this posting comes up ... I don't know if this is or will be the case of yours, But I just noticed today that one of my leaves on the very top ... errr second from the very top was very limp also. I pulled the leaf back just abit to get acloser look -- Found 2 new roots beginning to poke out, Abit strange for roots being so high ... But there they were
I'm hoping that your phal decided to do the same thing mine did. It's just doing' it's own thing.
Good luck and keep us posted