I'm not sure if this is a known bug, but any searches like "Dracul potion bug" or any other logical search wouldn't show results because they were too common.
Anyways when playing a Dwarf Monk I purposely let Dracul's peity drop to 0 by getting blessings (to intentionly get the penalty), I then used a potion. The peity drop sound happened, I stayed at 0 peity, but there was no punishment, and after raising my peity and using another potion to get 0 peity there still was no punishment (both were recorded in the peity log as well). This was after the boss was defeated, by the way I am using Chrome and windows 7 if its relevent.
There is a good chance that this bug could be used with other Deitys as well.