I believe it is an issue of probability. The Last girl in front of the bosses always shows up in the next or even the zone beyond that. Hence, the chance to encounter the elusive girl in zone 10 ist next to zero.
Thanks for the feedback! You were absolutely right: there was a bug in the spawn logic pushing the 5th girl to the next zone, making the one in Zone 10 impossible to find. I (hopefully) fixed this in the v0.1.1 update and she is now fully available. Have fun!
