This, I think, is a much stronger argument than "it is a plot hole." Zelda turning back cheapens the twist that she turned into a dragon and it lessens the sacrifice. To that I agree. Tears of the Kingdom's story is, generally, an unsatisfying mess, but that doesn't mean it's inconsistent or illogical.
I do still find Zelda's sacrifice very impactful because between the time I realized it and the time it was confirmed, I felt that I had truly unconvered a massive secret about the world. But then they wouldn't let me tell anyone what happened, which was annoying, and then they turned her back into a person, which made the permanence less impactful. But I still think the moment where she turns into the dragon is one of the best in the series.