Not sure its a bug. From the release notes:
"Monster behavior relating to stealthy players has been improved:
- Monsters should no longer "slide" while investigating noises and searching for players
- Sleeping monsters who are woken up by noises should no longer glitch out
- Investigate behavior has been updated
- Player characters should now display "hot footsteps" while sneaking in any direction, rather than just forward and back (this is a cosmetic change only, monsters could hear these footsteps before, but there was no feedback)
- Critters are now more easily "spooked" by nearby noises "
I think they just hear you run by much more easily. I invis farmed blockade buster, framework, and diplomatic impunity. I also used it to speed my runs through wiz0king and what not. It still definitely reduces the amount of attention you get, and the monster's are clearly still looking for you. But, unlike before, they raise the alert while doing it, which can make it a lot harder.
I assumed from the release notes that the change is WAI, and was meant to promote rogue stealth (w/ move silently), as the changes are supposed to be based on noise.