Phal gigantea

[IngieBee]

Hi, my phal has been in semi- hydro since the end of March and it just doesn't look happy. We've had a chilly spring, and it's only now hitting 80 degrees in the house. So I am wondering if the yellowed leaf is most likely;

1. Too cold?

2. Water got in the crown?

3. Illness (bacteria, fungal or virus)

4. Too much light (gets LED blurple light)

5. Other?

Thanks for any opinions and help

Top leaf grew about an inch since I got it, but but it seems like it stopped growing. Note, room was in the high 50's and low 60's dropping to low 50's over those few months before I thought to bring in a heater. I hope this isn't something it can't quickly outgrow??!!