Thanks again for all the replies!
Mucker, clear flats (all of them, including enamels) are very prone to this problem in high humidity conditions. The reason is the microsilica used as a flatting agent, which is hydrophilic. Either during the trip from airbrush nozzle to surface, or after it strikes the surface, it may grab extra moisture from the air and retain it in vapor form. The solution (sometimes) is to spray wetter and lighter coats. Can be a real PITK.
Contrasting Aaron, Mark, and Drew's comments, it is always possible to get a bad bottle, or a mislabled one. It is possible that quality control is suffering, somewhat, and that a CEO/CFO/BLBC (bottom line bean counter) needs a good hiding. But the consensus I get is that this is NOT (at least mostly not) a reformulation issue, which is a relief.
Almost makes me wish I was still working in a paint lab and could actually find out what was going on. My liver and lungs are glad that is not the case…