Miltoniopsis Leaves Yellowing After Repot

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The yellow leaves do look like the oldest leaves. They will turn yellow and die at some point. It looks like there is lots of healthy green growth and no pleated leaves. The roots look great.

One thing you mentioned is that the new bark got bone dry. Miltoniopsis don't like to be bone dry. So between re-potting and it getting too dry, that may have induced the oldest leaves to die off. I usually grow mine in mostly sphagnum moss with a bit of perlite, bark and charcoal mixed in. Several orchid suppliers say theirs thrive in moss so that is why I chose it. Milts like to always be partially moist so bark alone may be hard to achieve that. New bark doesn't absorb water as well for a month or so either. I use K-L-N on mine with no problems.

You mentioned you treated the bark with rubbing alcohol. If it didn't evaporate or get rinsed off before potting that may have burned the roots a bit too.