I agree that the paint probably needs a bit of the correct(???) solvent added, and note that the particles begin to settle unevenly as soon as the wet paint is allowed to sit. You might try situating the actual small surface more horizontally to better defy gravity, and apply thinner layers which dry faster.

But the most important thing to me when touching up an old frame is to minimize the area that gets touched up. I simply fill in the defects with the best color match I can come up with, making very sure not to overlap the surrounding good paint (which thus will expand the touch-up zone, and which always make a touch-up stand out badly).
For this reason, I almost never sand down a paint-chipped area, but rather I work completely within the confines of the original defect.
Also, with darker colors, I always err on the side of a too-dark touch-up color mix, and usually apply the paint only with the tip of a toothpick.

I am talking here about the smaller touch-ups, not large areas of course, and I should mention that the air is dry enough here that I don't have to worry about using primer or sanding down to bare metal.