If you have the Affliction talent Absolute Corruption spec’d, Corruption is literally permanent. Wither is a replacement for Corruption for Affliction, and thus Absolute Corruption makes Wither permanent. This is what makes Hateful Rituals in the Hellcaller tree so good since you get the 30% damage boost to Wither without the drawback of it having shorter duration since 15% less of infinite is still infinite.
And since the training dummies can’t die, it’s doing exactly what it’s supposed to do.