I'm not sure what's going on with Diplomacy, but for my ranged character, it does not function. I don't mean it fails. The skill check is successful, but nothing happens. The mob stays glued. Classic INQ build during sub29 leveling, v.high DPS thrower L29+.
Most recent observation was in R1 Strahd. Strahd glued himself to my ranged. Diplo. Success. No change. He's stuck on my ranged while a pack of melee beats on him. My ranged this life is on the tanky side. So, I just stood and tanked him. Diplo. Success. No change. Char died. Tanky doesn't mean tankNB: real diplomacy skill from a thrower, not inquisitive's automatic, though that also does nothing (packs of yellow success icons that never flinch off my ranged).
I know it can work, because I've seen it work on my Bard. Great DPS "for a Bard", but it's not a haxen build -- so B- or C+ maybe. My Bard's native skill is sometimes enough and I've seen mobs turn around after a success there.
The only thing I can think of as to the reason is that the Wiki data on Diplomacy is completely incorrect. Perhaps it was based on something once true, but that's not how the skill works now. The two parts that are incorrect are "causes mobs to attack another for 6s" and "puts you on the bottom of the hate list".
What I think is really happening is that it shaves off a certain fixed piece of your hate. If you're running a grade-B melee, no problem -- that's enough to loose agro at least for a while. If you're running a grade-A DPS ranged, it does nothing.
BTW, I'm not advocating ignoring the skill on characters that find it useful and especially not ignoring it on an INQuisitive. INQ is only high DPS sub-29. After that it's just a solid B at best. I can't say how or if diplomacy works now on L29+ INQ because I've not run it there since the nerf. It definitely did NOT work on INQ before the nerf, though. This isn't new behavior for diplomacy on high DPS.
I'm just finally giving up entirely on the skill for my ranged, though. Been trying to make it work for too long without results. I did find one very painful solution: Steel's Happy Time Coffee. My ranged pulls agro with -50% hate, but not with coffee. Coffee stacks it over -100% and makes you nearly agro-free. First agro will still stick until someone else touches it, but I've yet to pull anything off anyone while using it.
Coffee is annoying to use. Short duration and ugly debuff hiding in the buff line not the debuff line means you have to watch the hotbar not the game. And, it prevents rage and thus the FotW tree, too. That might be a mixed blessing. FotW is a lot more work for only maybe +15% DPS on my char.
If a dev reads this and thinks "not WAI", I'd suggest changing Diplo's success to include the complement of Intimidate's Hate+400; eg. a Hate-75 that lasts as long as Intimidate's version lasts. That would make even a sometimes-success useful to DPS. Coffee would become a choice or option for saving skill points, too, instead of "it's the only thing that works". Hate-75 will still punish DPS foolish enough to use a defensive stance, assuming that was the intent of including Hate+150 in the stance.