Answer
The missing grant can be a separate, not-obviously-related view specifically wired into that one screen's specific RMB action in the framework's own presentation-object definition, rather than the screen's primary view. In one confirmed case (a maintenance-module screen), two specific views not otherwise obviously connected to that screen needed to be separately granted before that particular RMB action would become available — this was only confirmed after escalating to IFS Support, since there's no screen or built-in way to discover this kind of view-to-RMB-action wiring in advance. The general lesson: granting broad screen-level access doesn't guarantee every specific RMB action tied to that screen is automatically unlocked, and a stubborn single-RMB-permission gap like this may require an IFS Support case to identify the specific missing view.