Not sure if this is a bug or working as intended. One of the heavy armor penalties is increasing the size of your detection area by 10% while sneaking per HA piece. It states sneaking but it affects the invisibility that Unnatural Movement (vampire passive) grants. Without any heavy armor pieces I can literally run into mobs without being detected, with a full set of heavy armor they will see me when I get too close.

Edit: same seems to go for Shadow Cloak. Mobs will spot me, they won't attack while invisibility is up but they give a red dot on my compass and will attack the moment Shadow Cloak drops. So is invisibility just a sneak with a very small detection range? I always noticed the eye thingie opens a bit when getting close to mobs but they never actually saw through it.