I was farming with a party using Fist of Thunder and Deadly Reach with Combo Strike. The %18 dmg rune from Deadly Reach worked correctly, as did the Breath of Heaven %15 dmg bonus. But when I switched from FoT to Deadly Reach I never got a %8 dmg bonus for 3 secs. I would attack with only FoT after using Deadly Reach once, too see if there was a dmg bonus that went away after 3 secs, but there was nothing.
I went into a game by myself and tested it out on a mob, it worked correctly. Maybe it only happens in a full party. Maybe the damage bonus worked the entire time, but didn't show up on my Sheet DPS.