After doing some AoE testing, I found out something interesting about the way
Roiling Blood works.
According to the tool tip, you'd expect it to simply cast Blood Boil and Pestilence at the same time. Following this line of thought, every time you cast Blood Boil, Pestilence should cast on your target. When you want to refresh diseases on your main target, you simply tab to a different target to cast Pestilence on it. However, this isn't what the talent does.
When you use Blood Boil with Roiling Blood talented, it selects
the closest enemy to your character with Frost Fever or Blood Plague. This means that if you're attempting to refresh diseases during AoE, you can't simply tab to a different target to refresh the primary target. You have to actually move your character closest to the target you want to Pestilence from.
I suspect that this is what many Unholy dps were seeing when using Roiling Blood to AoE, and reporting that it sometimes doesn't work. Blood wouldn't normally notice notice this thanks to
Scarlet Fever, and Frost avoids Roiling Blood in most situations.
I'm not quite sure if this qualifies as a "Bug", but it could cause problems when AoE-ing mobs with abnormal hit boxes.