Hard to say what it is due to at the moment.
The media you're using are fairly large chunks of bark, right? If the chunks are too large, then the plant can become dehydrated if the bark doesn't retain enough moisture.
Growers have various approaches to watering their phal. Maybe you can describe your approach - as in how do you do the watering? Do you use a watering can with a tip that delivers the water? Do the leaves become wet for a long time? Does the growing area have natural and gentle air-movement?
And this was the only orchid that developed black patches on the leaf, right?
Also - I forgot to mention - if an orchid is relatively warm or relatively cold, then putting water at a significantly different temperature onto warm or cold leaves can sometimes cause cell issues - leading to busting or deterioration of cells or patches of cells.
This might not be what happened. But just mentioning it - just in case.
For the moment - just provide comfortable temperature for the phal - for the leaves and the roots, and medium level light (not direct sunlight), and nice gentle air-movement. Monitor the situation. Check to see if the leaves become soft or mushy. If the patches dry up and harden, then ok.
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