The point I'd hoped to communicate is that its not possible to determine the cause of the behavior without a more detailed discussion of the environment. All attempts at explanation would be speculation.
In more general terms the command returns the active egress for a given port. If there are no vlans actively egressing the port, none will be in the output. "forbidden" on the other hand is not state dependent and will show up when the port has been so configured.
Use "show vlan" for similar information. The port in question will be present in the output beneath the vlan instance where port egress is active.
Note that "show vlan static" (no comparable show port egress command) will return ports configured for egress, regardless of port or vlan egress state.
I hope that's of more help and no additional confusion