The fertilizer might very well be the issue - so not rot from the outside, but killing roots. The time-release fertilizer actually responds more to temperature than to water. So if they got warmer, they got a blast. Phals don't need much fertilizer. I would suggest just repotting without the fertilizer, and let them recover. I would not do any "surgery" - a root that has lost its velamin can still hydrate the plant, and helps to hold it place firmly.
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