Help. Sudden Miltoniopsis Leaf Yellowing with Dark Outline

[realoldbeachbum]

I have had this Miltoniopsis Bert Field Crimson Glow for 4 months. It was mail order and came with almost no roots, but has thrived in low light, plenty of hydration, aeriation, and Kelp. Suddenly, on the largest leaf an elongated yellow swatch appeared from about 1/4 way up from base to about 1/4 down from tip of leaf. It does not go all the way to the base or to the tip. It has yellow center with thin dark edge. All other growths look healthy. It has had no overhead watering and is inside on grow shelf with a shop light about 3 feet up which is on about 6 hours a day.

I searched the web for pics of what it might be, but cannot find one that matches this (St Augustine has the best info!)

Do any of you have an idea as to what this might be? I have isolated the plant and plan to snip off the growth. Thanks sooo much, OB. You are my Orchid ER!!