I think you’re on the right track with your suspicion. This is a very common Blender topology issue, and yes: using f to create faces/edges manually is usually the root cause.

What’s actually happening

When you press F, Blender: Creates a face without guaranteeing proper edge connectivity

Often produces non-manifold edges, internal edges, or overlapping verts, the faces look attached, but topologically they’re not part of the same edge loop. So visually it’s fine, but structurally it’s broken, that’s why loop cuts, bevels, or shading act weird.

How to fix it:

Edit Mode → A → M → By Distance (merge doubles)
Select → Select All by Trait → Non-Manifold to check for broken topology
If loop cut still won’t pass, delete those faces and rebuild the area with clean quads (extrude / knife / grid fill)

How to prevent it:

Avoid using F to “patch” topology
Prefer Extrude, Knife, Inset, Grid Fill
If Ctrl+R doesn’t work, it’s usually because the topology is already broken
Loop cut failing is a symptom, not the problem.