Although this situation seems strange, the explanation is pretty simple. A lack of attic insulation directly above the area that gets moldy allows the ceiling surface to become cold during winter. The cold drywall encourages condensation, and this moisture fosters mold growth. I’ve seen this situation before and it’s especially pronounced in corners because air circulation is often slow there.
Your first step to get rid of mold begins in the attic. This will probably be challenging because there won’t be much room to work. As the roof frame tapers down towards the edge of your house space becomes short. That’s why this may be a job for spray foam insulation. Polyurethane formulations are closed cell, so they don’t allow internal condensation. You can remove existing insulation, then inject spray foam with tubing attached to the end of the spray wand.
To deal with the moldy ceiling area, you first need to kill mold. This used to involved bleach, but this method isn’t as safe nor as effective as more modern options. The best mold-killing product I know about is called Concrobium Mold Control. It’s a non-toxic liquid that you spray on the surface. As it dries it kills mold and mold spores by mechanical action, crushing them physically. Concrobium also offers residual anti-mold action, that prevents regrowth if the area ever does get damp again. Concrobium also offers the most effective mold stain removal product I know about. It’s called Mold Stain Eraser. Mix the power with water, then apply it with a sprayer. Mold stains disappear without scrubbing.