Separate Connected Surfaces says "The selected mesh is already a single connected surface."
I guess its the mesh like your're saying I didn't design this model from scratch. It is an imported STL from Thingiverse that I pulled into Fusion 360, mesh to BREP so I could modify it, then I used a SVG I created in Inkscape to emboss the feature then merged all co-planar faces with the merge tool.
Increasing the perimeter number until there's no infill in those little parts works around the issue.
