Unless rebirth is a major theme in your story, if you want readers to believe your characters actually die, it’s almost universally recommended to keep characters dead.
When you bring a single character back, you are then an author who brings back dead characters.
As to your specific question, I think the sadness of a dead character lingers and sweetens as time passes. Immediately following up with hope that they’re still alive only allows for the initial sadness and then tells the reader to stop being sad.
"...the one with whom he so sought to talk has already interceded for him." -The Master and Margarita