Marvin's Blood doesn't work as intended, giving +0,75 extra mutation rate points instead of +0,25..
As you can see from the video, Marvin's Blood doesn't respect what the in game description says: "Increases Mutation Rate when hitting survivors with Tentacle Strike by 25%".
When a survivor IS Contaminated, as you can see from the Dbd wiki, it should be: "Hitting contaminated Survivors with a Tentacle Strike: +1 Contamination Points".
Nemesis T-Type or "The Nemesis" is one of 32 Killers currently featured in Dead by Daylight. He was introduced as the Killer of CHAPTER 20: Resident Evil™, a Chapter DLC released on 15 June 2021. He originates from the video game franchise Resident Evil, specifically the 2020 remake of Resident Evil 3. The Nemesis is an unrelenting Killer, capable of attacking at mid-range with his tentacle, while receiving support from Zombies roaming the area. His personal Perks, Lethal Pursuer, Hysteria, and
Marvin's Blood, from the game's description, should give +25% extra, which should be 0,25 mutation points. So you should receive +1,25 mutation points in total after each hit.
But this is NOT what is happening in game:
6 mutation points are required to reach tier 2, but if I hit Dwight 2 times, I'm over half the bar, instead of before it at 2,5 mutation points.
This is because the game always gives +0,75 mutation points flat after each hit, which corresponds exactly to the bonus 25% that you should receive when a survivor is NOT contaminated as then gets contaminated: from the Wiki: "Contaminating Survivors with a Tentacle Strike: +3 Contamination Points". 25% of 3 is 0,75, but as the game's description says, you shouldn't get always +0.75, but 25% of 1 point in this case, which is 0,25.
In game after 2 hits I got at 3,5 mutation points, which is 1,75*2, which goes the half bar point.
So Marvin's blood is bugged/ isn't working as intended, or the in game's description is wrong.
This is Marvin's Blood values extracted from the Dbd wiki:
Marvin's Blood is an Uncommon T-Virus Add-on. Nerf: reduced the Mutation Rate Gain bonus from +1 point to +0.75 points. "Marvin" refers to Marvin Branagh, Senior Police Officer and Lieutenant to Leon Scott Kennedy at the Raccoon Police Department.He helps both Leon and Claire Redfield crawl back out from under the shutter door outside the East Office when they are attacked by a Zombie.Afterwards, he occasionally provides them with bits and pieces of information, while they continue exploring the

Comments
I now put the video link , my apologies.
Pretty sure this is a case of the description being wrong, not the effect. If you check the wiki, you can see that every single add-on modifying the contamination points works that way. Actually, the description isn't wholly wrong, it's just wrong for any instance of hitting an already contaminated survivor, since that action gives fewer contamination points compared to hitting them when they aren't already contaminated.
The devs often simplify descriptions when the technical aspect would be too complicated (percentages in this context are easier than contamination points).
The wiki has a different philosophy for that stuff, they prefer the technical explanations, as those reflect what actually happens behind-the-scenes (see Monitor & Abuse on the wiki, for example) and, as we can see here, avoids issues with percentages over flat numbers.
Theoretically, if the devs want to stick percentages here, they would need to rephrase the description to mention two percentages: one for hitting non-contaminated survivors (25%) and one for contaminated ones (75%).
No, because if you hit a survivor already Contaminated you get by default only 1 point, so you should get only +0,25 extra from Marvin's blood, as by description.
I hope it's the description being wrong, because otherwise the add-on might even have been nerfed for no reason if the devs didn't actually read the real effect in the code. I really doubt that happened, but you can never know.
Here we go, in the PTB, after aknowledging the issue, the description got changed, so I was right.
Fair enough.