Download a newer version of Blender, with new Boolean library. I'm not just sure in what version 2.91 or 2.92 ...
old 2.8x issue
old 2.9x Fast (2.8 method)
new 2.9x Exact
It is cut out in inside normal direction (red color when Face Orientation overlay enabled).
If you still need to work in 2.8 you would have to use Solidify modifier or other tech to create non-manifold object as you mentioned.