I was testing weapons in the Simulacrum and than noticed that my max rank Pressure Point, which should give a 120% boost to damage, was giving only 75%. I also tested it with Spoiled Strike, Killing Blow and Sacrificial Pressure, and they all gave only 62.5% of the bonus shown on the mod.
Example:
Using a Cerata's charged throw recall explosion (base damage: 2108.8 toxin) on crawlers (toxin does neutral damage):
Unmodded: 2108.8 damage
Using Pressure Point: 3690 damage -> 2108.8 * (1 + 0.75) = 3690.4; Also shown this way in the damage UI
Adding Spoiled Strike: 5008 damage -> 2108.8 * (1 + 0.75 + 0.625) = 5008.4; Also shown on the UI
Adding Killing Blow: 6590 damage -> 2108.8 * (1 + 0.75 + 0.625 + 0.75) = 6590; Killing Blow does not increase damage shown in the UI
As shown in the examples above, all these mods have had their damage reduced to 62.5%
Pressure Point: 120% * 0.625 = 75%
Killing Blow: 120% * 0.625 = 75%
Spoiled Strike: 100% * 0.625 = 62.5%
I also tested this on other melee weapons, and the issue persists
Also to note:
I checked the melee weapons that I own, and noticed that the damage values listed on the Warframe Wiki are also 62.5% of the values listed in-game:
Cerata's charged throw recall explosion is listed as 1318 toxin damage in the wiki, however 2108.8 toxin damage in-game (2108.8 * 0.625 = 1318)