i also have seen this problem happen but its not new, infact i have been complaining about it since Beta 2013.
it is not only happening to penetration, it also happens to my defenses as well as my damage.
Grand Master crafter, all styles and all furnishing plans known pre U41.
Vamp and WW bites available for players.
Shoot me an in-game mail if you need anything, happy to help!