I've noticed this and its quite annoying. As Post Mortem is UB's primary anti-creep ability (Ooze might work, but I don't like that skill's suicidal tendancy), I've always found that upgrading to Plague is counterproductive. Frankly, I would rather have Post Mortem than Plague, so why should I bother putting skill points into an ability that essentially removes Post Mortem?
Plague greatly weakens UB's combat effectiveness due to this bug. Please fix it.