Cyrus confirmed her and Hemsworth's engagement earlier this month.
She said at the time: "I'm so happy to be engaged and look forward to a life of happiness with Liam."
Hemsworth's marriage plans have apparently upset his high school sweetheart Griffith, who told CeleBuzz that she was blind-sided by the news.
"I'm not going to lie, I'm a little surprised," Griffith admitted, adding: "[Liam's] still someone I care about."
Griffith went on to say that she was disappointed to see Hemsworth moving on from their relationship, which apparently lasted for six years.
"Of course it hurts seeing someone I spent so many years with moving on, especially when I learned from the media," she said. "But at the same time if they are in love and are happy, why not? I'm very happy for him."
Earlier this week, Cyrus denied cheating allegations after she was photographed swimming with male friend Cheyne Thomas.
Cyrus and Hemsworth have dated since 2009.