SOS leaf discolouration Phal Belinna Red Apple

[YokaYampi]

Requesting a little bit of help!
I got this BELLINA red apple in September, being new and eager I didn’t mind the odd black thing on one of the leaves despite asking the grower and them giving me an odd and vague answer I still bought her. Looking back now, much more experienced I know it was a bad idea, and I’m definitely not going back to that grower, but for now I’m hoping it was nothing serious an the plant can outgrow it in a few years and loose it naturally.
The issue is in the last few week or two I’ve noticed this leaf having a yellow discolouration starting which is worrying me... she’s currently in an orchidarium with a PC fan and a rep tails heating mag that has a tray for black pebbles that the orchid sits ok top of, the fans are on regularly, the heat stays at 22c daily and doesn’t get below 19C nightly. Humidity is usually at 55-70% and grown under LED lights
The only thing that’s changed in the last week is that I forgot to turn the fan on one day so the orchidarium didn’t have solid circulation for 12hrs.. but I doubt that was it
What should I do? What can cause this type is discolouration?

Phal Bellina - Album on Imgur