I think time and frequency are two important things. One of my favourite mangas consistently does fakeout deaths to the point where I genuinely wonder if anyone going "omg they're going to die" is reading the same show. No one dies outside of minor characters and backstory. Which is fine, they could have high stakes and make me worry about the character without ever killing one off, but the constant "they're dead, grieve for them, oh wait no they're not" gets tiresome. If you want to emotional impact of a fake out or undone death, don't over do it or it becomes a joke or worse boring. Like any trope, the more it's done, the more desensitized your audience becomes to it.
And also the time between the death and the reveal/ressurection. If it's next chapter/update, your reader hasn't had time to process it or the impact. And just good writing. YGO series do this quite well, actually. Most YGO seasons will "kill off" pretty much their entire cast in the last season to prove how scary the big bad is, and these scenes can be heart wrenching, even knowing that they're all not really dead and will come back. Good, emotionally impactful writing is good emotionally impactful writing, even knowing the stakes of lack of. But also, when YGO does it, those characters won't be back until pretty much the end of the season which can be the better part of a year for some of these shows and the amount of time they're "dead" for helps it feel less like a throw away thing. I know these characters are coming back, it doesn't stop me crying over the scene.
So I think generally, the death should still feel like a death and be written like a death and should have the impact of a death, however that impacts the characters. And the same, I don't think resurrections should feel cheap either. Think about any other situation where you lose someone. A break up or losing a friendship, things just don't automatically go back to how they used to be when you meet that person again and I think a resurrection should be similar. Yes, you're character might be happy they're back, but we process things while we're grieving and it can change people and make you look at your relationship differently. And now that person they've come to terms with no being here is here again.
And that's not going into how near death or actual death experiences can change people. I can't remember which show it was, but I saw a show with a bunch of characters who couldn't die. One became ridiculously reckless and carefree only to have it hit her too late that can't die doesn't mean can't even up in awful situations while another was developing PTSD from all his deaths that he came back from.