is this a characteristic of erratic?

TBH sometimes they just don't "witness it" despite being in plenty proximity to it. I've had family members miss the moodlet/recognition before despite being in the same household.

I don't think traits have anything to do with the sad moodlet of the death of a sim. I say this because my sim was at a gym when someone died. I decided to have her plead with the reaper for the dead sim. She failed because she didn't know him, and she was even standing within eyeshot of the dead sim laying on the floor. After failing she left the lot, she never got the negative sad moodlet. She did not have the erratic trait.

It's probably not the trait. I've never played an erratic sim but I'm pretty sure I've had sims before that should have been sad about someone dying but they didn't technically "witness" it, so they weren't.