IIRC there's some config item somewhere that sets if apps will see one screen as "full screen" while using surround mode.
Blast me if I know where.
Nowadays, I usually set it up as 3 individual screens ("activate all displays" in driver) when I'm not racing, and only use the "surround" mode when racing.
This is mainly due to the Pinball Fx2 that cannot tolerate the xxx-wide desktop, tho