It is not the paint. I can actually see particles of sand (they are beige-colored) attach themselves to my wet surface, say a dark blue surface, over the course of a few minutes. I've been painting for over 40 years and taught college/univ. art (drawing and painting) for 30. I know paint. I'd post a picture of these particles on a wet painted surface but would have to send one from my phone to my laptop. Clearly these particles are sand, along with some other debris remaining from the building's sandblasting (pulverized pieces of brick, wood, dried paint and bitumen—"pitch," which was used as a floor covering/adhesive back when buildings like these were built). I've already identified these contaminants and they are external to the paint.
I opened my windows yesterday and the humidity in the room increased to about 40% (I have a humidistat in my studio, although not a very good one). I also directed a box fan away from my immediate work area toward my suspended air filtration unit. The higher humidity seems to be solving the problem—which supports my theory, as unlikely as it seems, that the very low humidity in the space has been causing my wet paint to attract these particles. I'll know more as the warm days continue. My studio has been bone-dry since last fall.
I wish you all could see this problem in person! You'd plainly see what I'm talking about.