How is 1539 tooltip less than 20% of your weapon damage? You have 7000 weapon damage?
Cyrodiils light got reduced from 25% to 15% aswell. Morag Tong also got adjusted.
Classic ZOS: "Hey guys we gave you all the old item sets back with horrible rgn bags - also all sets worthwhile using got a nerf just in case..."
Cyrodiil light, whatever, don't care about the set.
Morag Tong set, as it stands, will effectively be used by 0% of the competitive builds in PvE. Simple math: even if poison damage was to make 50% of your total damage on a sustained fight, that means a 10% increase of poison damage effectively increases your damage by... 5%. And that's supposing you don't lose a 5th piece bonus, such as Hundings/NMG/TBS. A few DPS parses thus far have shown that my poison damage on my stam DK makes for about 35-40% of my total damage dealt on most boss fights. While I do not run Viper's Sting (classical Mael daggers), it's still pretty unlikely to be good enough to compensate for a global loss in damage, both poison and physical.
So, if not even stam DKs use it... who will? 15% on Morag Tong was fine, but all you had was a bunch of "OMG STAM DKs OP NURF" with literally 0 math to back it up.