When Jack rushed toward Master after finding out what Master was up to, Grace was confused at first. She only saw Jack flying toward the divine heritage. She couldn't see Master. But her keen eyesight noticed the seal around the divine heritage was peeling off little by little. She understood then that something was wrong.
Her Angelic Possession had just come off cooldown after using it to fight the happy dolls earlier. She immediately activated that skill and followed Jack to the divine heritage.
When Jack suddenly turned back to save Talia. She was conflicted about whether to follow Jack back. But she quickly decided that it was better if she left Talia to Jack and she went to protect the divine heritage instead. So, she continued onward.
She couldn't see Master. She didn't even know it was Master that was there. But after coming into range, her mana sense quickly picked up on the invisible person.