If you globally map "Open Front Row" to Menu (held), that button is no 
longer available to Front Row's default settings, which has "Open RB 
menu" mapped to that button. RB automatically adjusts your FR mapping 
table and moves the "Open RB menu" to a button that doesn't yet have 
any action attached to it. Like - for example - the "Plus (held)" 
entry. Then, if you hold down the Plus button on your AR to scroll, RB 
will no longer directly forward all events to the action mapped under 
"Plus", but differentiate between "Plus" and "Plus (held)" and route 
the event to the action mapped under "Plus (held)" when you press the 
button for some time. A short click will still be routed to the action 
under "Plus".

Possible solution: 
Go to RB > Prefs > Mapping > Front Row and ensure that "Open 
menu" (and no other action, either) is mapped to "Plus (held)" and 
manually remove it (by selectin "-" from the dropdown), if there is.

Best regards, 
Felix Schwarz