A hotfix has been released that should allow all SV hunters attacking a target to benefit from Noxious Stings. The issue had been that only the most recently manually applied Serpent Sting on the target would offer the 10% extra damage dealt from Noxious Stings. Any older Serpent Stings on the target would not confer the damage bonus to the hunter(s) who had applied them. This was obviously a problem whenever you had more than one SV hunter in the raid because only one of them could have the damage bonus at a given time.
Multiple hunters attacking the same target should now see the proper benefit from Noxious Stings.