Nothing special here. All ARs with few exceptions come with a muzzle fixture which suppresses flash.
It was only during the lifetime of the AWBAN (1994-2004) that new production ARs did not have this muzzle fixture.
No, not unusual at all. My point was, IF there was another shooter, what would be his purpose?
Only two that I can think of: shoot someone the other guy couldn’t (which doesn’t seem to apply here) or
Occupy cops for a while so “main” shooter does more damage. Maybe he was to fire a few bursts then leave? Point is, his “apparently” has flash bursts to see while Paddock’s didn’t. It would have taken longer to figure out the shooter was much further up.