I have had this bug for awhile, and finally found it's cause.
Apparently while wearing a combination of Leeching Plate, Soulcleaver, and the Oakensoul ring, and loading into a zone or logging out/in, the ESO plus buff that gives your character the 10% bonus didappears along with its benefits.
I have found that waering the full set's bonuses together triggers this, but not Oakensoul+ leeching or Oakensoul+ Soulcleaver, nor Soulcleaver+ Leeching, but the combination of all three together.
@ZOS_Kevin Can you please forward this to the devs so they can be aware of this bug?
@Djennku, PCNA.
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!