Phalaenopsis base is turning yellow

[euplusia]

You have a healthy root system and one healthy leaf. I don't think that the center of the plant is destroyed by rot. So the plant can recover, but it takes time.
What looks yellow is the base of the previous leaves. I cannot tell you why the plant lost it. It happened more likely due to stress than to a fungal infection or overwatering.
Do not disturb the plant anymore, do not repot, just continue your care and be patient. Best is indoor growing, medium shade, temperature of 20-25°C and watering once a week.
Last but not least: Welcome to ob.
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