Help. Sudden Miltoniopsis Leaf Yellowing with Dark Outline

[SouthPark]

ROBB - was this plant growing under the conditions it was growing in (under those same lights) for approximately 4 months?

If that is the case, then some consideration would need to be made about the reason for the abrupt development of that damage on that leaf there.

ES could be onto it about temperature and/or light level. If some particular hot day came up, then it could affect the plant. Not sure about why that particular leaf though.

Could also see if there's any attacking bugs.