The change was pretty sudden and jarring, they probably should have dedicated a couple of pages, or even a couple of panels, to montage their weight loss.
I genuinely thought that this one could have been a story that would have the weight loss journey be a bit longer so that we could have more moments between Erika and MC...
I can see he was just giving you a playful hard time on choice-of-word, since as it's used, fiancé is the masculine use, while fiancée is the feminine one. Any issues you want to bring up, bring it up with the French.