I don’t grow any masdevallias, but that looks like false spider mite damage, some species of the genuses Brevipalpus or Tenuipalpus. They are tiny and you can’t see them without magnification but if you swab the underside of a leaf with a white tissue they appear as a rusty powder. You can spray the leaves with soapy water or isopropyl alcohol, which cuts down the numbers. Air circulation and/or increasing the humidity will also help, but it sounds like you have that covered. There are stronger miticides but I haven’t tried them. If you have them on multiple plants there are predatory mites you can try.
PS it’s a beauty!
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