Hey, when casting scare on sand trolls, they'll do the save, but won't actually get feared. There doesn't seem to be any event messages saying they are immune to fear, so I assumed it was a bug.
It's a side-effect of them being immortal (https://nwnlexicon.com/index.php/SetImmortal).
Not much we can do about it, but also it's pretty much the only mobs that have this mechanic, so fine as is.