Phal leaf turning yellow after new roots

[Ophiel]

Hello,

I'm still a novice with phals, having killed 4 in the past and learning progressively more from the forums on orchidboard, enough to troubleshoot and keep the remaining 3 alive.

One of them is a NOID phal that I've been growing for about a year. It's been doing relatively OK this whole time. I water once a week, and feed weak orchid food every other watering. No growth or anything since the last flowers dropped months ago.

Then just a month ago (spring in Australia), I was thrilled to see it sprout some roots and a new leaf at the same time! Then just last weekend, I noticed the lowest leaf turn yellow very rapidly. By today, it is almost completely yellow and droopy. All the other leaves seem fine, even the new one. Is this normal? Does this have anything to do with the 2 new roots growing right above the leaf? Should I be worried, and if so, what should I do?

Attached photo for reference: