devtrackers.gg
Yeah, this is a bug. The decay isn't actually applying a modifier to action speed, but its modifier has been incorrectly folded into the action speed total as a cheat to implement it. That leads to both this display bug and also the fact that being in the decay will let you move while frozen (because all actual modifiers to action speed are being ignored while in the decay).
I'll fix this up for a future patch.
also why does tar dropped by monsters slow you as a jugg with unstoppable/with kaoms roots
Tar reduces movement speed, not action speed. Kaom's Roots only protects against the later. Unstoppable does both.
However, those modifiers only prevent it being reduced below the base value. If you've already increased movement or action speed way above the base, they can be brought down a bit, just never below the base speed.