I don’t really understand your Boundary Conditions, but your stl geometry is completely closed up. Even what you call inlet.stl and outlet.stl are used as no-slip wall.
The zmax boundary is located at z=0.401 m but you apply an outlet BC at z=0.4m. This is OK for a coarse mesh, because the BC plane coordinate snaps to the closest grid point. This doesn’t work with fine mesh (DZ=0.5mm, smaller than the difference between zmax=0.401m and z=0.4m) because now the z=0.4m plane is located inside the domain and is not a boundary anymore.
There is also a conflict since you define both an outlet and a wall at z=0.4m