Why are spoiler panels 5 and 10 locked in high speed flight on the B777?

Question

From the Boeing 777 FCOM:

Spoiler panels are used as speedbrakes to increase drag and reduce lift, both in flight and on the ground. The spoilers also supplement roll control in response to control wheel commands. Spoiler panels 5 and 10 are locked out during cruise, depending on altitude and airspeed.

What is the reason for locking out this particular spoiler pair?

Answer 1

To prevent tail buffet, reportedly:

In the NORMAL mode spoilers 5 and 10 are locked out during high-speed flight to prevent tail buffet.

That's from a study flashcard that was posted on chegg.com. But I could not corroborate it.

However, during the 777's flight testing campaign, tail buffeting did need fixing:

... Adjustments were still being made to the aerodynamics, particularly to cure some tail buffet [...]

_{— Birtles, Philip. Boeing 777: jetliner for a new century. p. 26.}

Spoilers 5 and 10 are in-line with the stabilizer's tips, and a paper confirms the spoiler interaction, in general:

... separation over the wing may exist and also interact with the spoiler. Besides, the wake of the spoiler has to be accurately resolved, as it may be impinging the horizontal tail and trigger tail buffet leading to handling issues in some cases.

_{— Brooks, Timothy R., Gaetan K. Kenway, and Joaquim RRA Martins. "Undeflected common research model (uCRM): an aerostructural model for the study of high aspect ratio transport aircraft wings." (PDF)}