This chapter actually directly contradicts something Iris says in chapter 11 about Mizuki, where her foot is injured by a thorn in the trail and her injury does not regenerate, implying she isn't a vampire. So it's either a minor plot hole or Iris is either misremembering or lying.
Edit: Looking at the flashback sequence more carefully, I'm convinced that it isn't a mistake on the author's part. The events are showing Iris breaking a shoe and then both of them going barefoot, which prompted the thorn incident. This probably means it will come up again.