This is what the manual says and I quote:

"To use the OSD, the units must be connected to your display device using either the COMPOSITE VIDEO OUTPUT or S-VIDEO OUTPUT jacks. You may also view the OSD using the COMPONENT VIDEO OUTPUT, but the picture may blank temporarily when you activate the menu system. If both s-video and composite video signals are input into the Model 990, the OSD can be viewed only through the S-VIDEO OUTPUT jack".

Did I just misunderstand what I read?