It seems if you are wearing a legendary flak vest and get a hit that hits both the torso and some unprotected bodypart, the torso takes a full damage hit.
Leg+torso hit with non-mitigated torso hit: https://steamuserimages-a.akamaihd.net/ugc/1831278003378754665/6E81447882089D8C9BCAEF6023D51ABA44DFC56C/
180% sharp protection on torso: https://steamuserimages-a.akamaihd.net/ugc/1831278003378705336/5AD47BC1CCE23AADE8B43425591BEB7CD44ECA84/
180% sharp protection against a rat scratch should have a 0% chance to do full/sharp damage on torso, yet the torso takes a full damage hit if the unprotected leg also got hit.
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It is worth noting that I did these tests because someone said that flak vests do protect bionic arms but not flesh arms, which itself sounded like a bug/oversight to me and I tested this. "It's worth noting that a flak vest will protect bionic arms, but not fleshy ones."
(source: https://steamcommunity.com/app/294100/discussions/0/5015323029412694252/#c5015323029412993274)
It seems like they took full damage hits though: https://steamuserimages-a.akamaihd.net/ugc/1831278003378738240/5FC968A6EE0B00555FC6C6C604FA63DE29EB92EF/
On this one it did not take full damage: https://steamuserimages-a.akamaihd.net/ugc/1831278003378789897/60E0D9B967D20888B2D26E693846BC1D6E2277F3/
Flak states to protect torso and shoulder and it seems like it still possible that both can take full damage with a multi hit. I don't know if multihits can just always do full damage, even if ALL of the parts are protected or if bionic arms are just treated special to not have the "arm" just a shoulder part that is indeed protected by a vest. It might also just roll on one of the body parts first, take the damage from that and spread it over all. This would explain why one hit on torso + shoulder did full damage and the other did not. Maybe the mitigated one rolled torso first and the full damage one rolled arm/shoulder first? Or the full damage one rolled arm, which is not protected, but is still noted as a shoulder hit, since that is the only body part noted for bionics?
I hope this was clear enough. I am a bit confused myself and can just guess if there is a design decision for bionic arms to name them "shoulders" but not have them protected/still cover multiple body parts or it the multi hit bug is just triggers on more than just protected+unprotected. I really hope bionics are not protected by flak vest just because they are "shoulders" on the body part chart, since they are obviously full arms down to the hands.
Whatever is going on with armour ignore seems also a bit more complex and I think testing alone does not suffice here without knowing the code, since I got full damage and mitigated hits. Full damage on torso should not be possible and if bionics are indeed treated as arms, mitigation should not be possible (except if it was a shoulder hit?).
Both DLCs, no mods.
If something is unclear, please ask. I can do more tests, if you have questions or can try to reword this. I am mostly guessing here and just try to show something is weird and try to communicate the "it's possible that fully protected body parts still take sharp damage hits" + "I hope bionics are not protected just from shoulder armour".
Thanks for the bug report, I think I get what is going on. It seems like if there is an attack that targets two locations, it isn't taking into account armor at all, and perhaps if there is a direct one location target to bionic arms, it does get flak vest protection. I will put this in for some deeper investigation, thanks for taking the time to look into it further.