The breakdown of the forces associated with a propulsion system into “thrust” and “drag” is to some extent arbitrary. It can indeed be confusing sometimes and good booking is important in the end.
Spillage drag is related to flow conditions above and below the mass flow ratio at which flow separation occurs at or near the cowl lip. This can be infleunced by the locked-rotor condition.
There should be many NACA/NASA studies on this topic giving more background.