What is this pattern of discoloration around my ceiling AC supply vent, and how can I stop it from happening again if I repaint?
Background: In September 2016, I had an air conditioning system installed in my house (built in 1893, located in Massachusetts, USA). This included all new ductwork (we have hot water heating so there were no pre-existing ducts). The air handler is located in the attic and there are 5 supply vents upstairs and 3 downstairs, with a central return in the upstairs hall.
Around the end of 2017 I first noticed some brownish discoloration starting to form around this vent. It has continued to expand in 2018 and 2019.
This type of discoloration is only visible on this particular vent, and none of the others. This is the vent for my living room and it is the largest of the 3 downstairs vents. This is also the only downstairs vent mounted in a ceiling paneled in fiberboard (masonite, I think). There is also a beam and wall near this vent, which may be impeding airflow, especially in the directions where the discoloration points. I doubt there have been any leaks/drips above this area.
I suspect the staining is some sort of condensation; could it be happening in the fiberboard, caused by over-cooling because of the nearby beam and wall?