Since 4.0 the basic rules for DoT behavior has changed. When a DoT is applied, the following rules are applied:
All player stats are "snapshotted," including all bonuses received through buffs and procs, and any losses to stats from debuffs. This means that the DoT will not benefit from any procs
unless it is recast.
The DoTs damage will update with boss debuffs on a per tick basis. For example, if a DoT was applied to a boss before the 8% Spell Damage debuff was applied, and the 8% Spell Damage debuff was applied between the 2nd and 3rd tick, the 3rd tick will begin to benefit from the debuff immediately.
The only known case where the above is not true is with the 5% Crit Debuff (Shadow and Flame/Critical Mass). This debuff follows similar rules to that of player buffs.
Источник:
elitistjerks