The Maroon 5 frontman and model Behati Prinsloo's happy news was reported last week.
Adam has been linked to a number of beauties in the past, including 21-year-old Danish model Nina Agdal.
The singer and Behati briefly split in March, and Adam was seen taking a holiday with Nina in June, just weeks before his engagement to Behati was announced.
New York Post is now reporting Adam, 34, let Nina know about his proposal to Behati by sending her an impersonal text message.
The publication also claims the model is left "heartbroken" after hearing the news.
Representatives for all parties declined to comment, but earlier this week Life & Style magazine said Nina is doing her best to move on.
"She didn't see it coming it all," a source said.
"Her dad came out to LA for a few days, then went with her to Mercedez Benz Miami Swim Week."
Another added: "She's doing just fine. Her dad was in, she spent the weekend with friends and going out."
Adam and Behati, 24, have been dating since 2012.
They reportedly started seeing each other shortly after Adam split from Victoria's Secret beauty Anne Vyalitsyna in April last year.
Behati seems giddy about the upcoming wedding and recently gushed about the romantic proposal.
"I am so happy and excited and still can't believe it's true!" she said. "It was a one-knee thing! It was serious, it was very old school!"