Phal leaves suddenly turning yellow & spongy

[goulstone]

I received a beautiful kaleidoscope phal in bloom from Mom a week ago. Had a small sunburn patch on one of the lower leaves -- not a big deal, however, I'm alarmed at the sudden softening & yellowing of the 2 newest leaves which up until a week ago had been firm and green. The situation is changing fast -- last week the plant seemed OK and now I'm afraid it is terminally ill

Hoping someone can tell me what's the problem and if there's anything that can be done. Or how to avoid this happening to other chids in the family. (already isolated this one to avoid spreading diseases)

potted in bark with lots of air circulation