On Rhino 6 SR30 and getting a weird behavior for the preview mesh quality settings when paired with custom preview components.
On re-computing, what you see as an output from custom preview, is as if “disable meshing” is activated.
Switching between settings seems to affect the default shaded preview geometry, but not the geometry coming out of custom preview.
Disabling and enabling the custom preview components seems to resolve this, till the next re-compute.