Yellowing leaves on Oncidium

[Shamu]

Hello all! I have a 6 year old Oncidium Twinkle 'Fantasy' in my care. About a year and a half ago, the perfect storm of low light, too much water, and aging media left the plant vulnerable to rot. The older psuedobulbs yellowed and became soft and mushy, dropping leaves. In attempt to save the plant, I removed all the yellowing and mushy psuedobulbs, leaves, and roots; repotting in a fresh bark mix.

Fast forward, the plant is still recovering and has been slow to bounce back. It began sending out new growth this past January, and things were looking up. However, in the past few days, the plant has started to yellow again. The tissue is still firm. Nothing has changed in the care. It is currently on a S facing windowsill with some extending LED light for ~6 hours a day. After a brief scare of underwatering (in my attempt to prevent further rot, I held off watering for too long, leading to a brief week of being underwatered), I've been closely monitoring its moisture levels with the bamboo skewer technique, using distilled water (the pictures show wet media because I watered it right before the pictures). I do not currently fertilize.

Does anyone have some helpful advice or thoughts as to what may be causing this downturn in health? Could it be temperature related by being on the windowsill? Could the brief underwatering period have killed off the root system and the consequences of which being this yellowing? Is it under-fertilizing? Might it be some sneaky pest munching on my beloved plant? How do I test out these different hypotheses without stressing the plant further?

Thanks for the help!