Hi,
I’m printing mainly flat shells and I witness this effect - the surfaces facing to the build plate when they are close to horizontal (below 30-40 deg) and also if there are supports in the area appear covered with cured resin. All details in this area are smeared, parasitic cured resin is thicker around the supports. See this conversation in the foruma:
The solution is simple in your case - just rotate the detail to be 60-70 deg to build plate and ensure you have enough supports.