Reincarnation isn't the same thing as resurrection. Mera and even Hellbram were able to resurrect people but their souls were not in perfect condition.Not just Elaine, But Rou died and came back as Oslo.
Then there was Mera who could revive any regretful soul.
SD's curse could revive Mel countless time, and her Graces bring AAs back from the dead indefinitely.
Finally, Mael used a spell on Der and Oslo which could give them another chance to live.
Although it's a good decision not to mess with the concept of death too much, such a retcon could work better coming from a new 4KotA specific character. Gowther has seen enough revival in previous 3000+ years to consider it a common everyday life phenomenon, rather than an impossibility!
Besides Elaine's case I guess Mel and Elizabeth's situation count too.