Mizuki didn't know why Seigi was suddenly so adamant on calling them all together, although she got an inkling of an idea once she had been informed about what had occurred in the camp. If Magi had been injured it wasn't surprising that Seigi would be on edge, given how close she was to the pink-haired spellcaster.
That said, she still found Seigi's behaviour... odd.

Where'd you get your hands on a map for the tunnels? (She opened her atlas and flipped through it.) These maps only cover the surface.
Still, if they did have a map of the underground it at least made the tunnels viable, even if Mizuki still had her objections. Unfortunately, whatever it was that had happened made Seigi unlikely to listen, given how she pre-emptively dismissed any concerns the rest of them might have regarding the tunnels.

I agree with Yugam. New intel means we need to reconsider our options, not rush in carelessly. On top of that... are you feeling all right Seigi?
Magi was one thing, having gotten hurt in the incident, but the heroine's attitude was definitely different from usual. Especially the part about being "past redemption"...
@Cleo Looker,
@Magi Heart,
@Seigi Ling Ling,
@Yugam