Steps to reproduce:

Opponent cast Kozilek, the Broken Reality.
Forced me to manifest cards, which I only had one. I manifest Herigast, Erupting Hellkite
On my following turn, I went to cast Petrifying Meddler, and I was given the option to Emerge this (even though Herigast was still facedown)

1. Platform: PC
2. Was able to emerge Petrifying Meddler
3. Should not have been able to cast spells with emerge while Herigast is facedown.
4. See attached.
5. No workaround, other than just not casting with emerge.