Current other bonuses I have: The base 1.2x and cold mages + Memento mori => 1.2535x
Without PotW:
Shown vulnerable damage: 161.3%
Base from gear + paragon: 73.7%
((1 + 0.737) * 1.2 * 1.2535) - 1 = 1.6127954, which rounds to 161.3% very good
With PotW(being outside town is necessary):
Shown vulnerable damage: 185.3%
[Shown] Base from “items and paragon”: 89.7%
73.7 + 16 = 89.7
((1 + 0.897) * 1.2 * 1.2535) - 1 = 1.8534674, which rounds to 185.3% and checks out.
If it was instead multiplicative, it would be ((1 + 0.737) * 1.2 * 1.2535 * 1.16) - 1 = 2.030842664, which would round to a shown total of 203.1%
The secondary effect from the Desecration glyph shows on the stats the same way(in this case 15%), a flat increase on the “You have +__._% of this stat from items and Paragon.” section, and thus I assume it is also bugged in the same way.