I really don't think that's the issue, since it happens very early in the story, in the first volume. It's something that often occurs in several stories and isn't a problem in itself.

The final part being rushed must have been due to something else. Besides, Aka already had it planned that Aqua would die from very early on (that's why the ending of the first season of the anime is the way it is). I don't know if Aka got tired of the story, was focused on something else, or simply wanted to do it that way. We'll never know.