A source close to Miller told Hollywood Life that the 26-year-old rapper is upset over Grande's engagement to Davidson whom she started dating in May.
"Mac is devastated that Ariana may have become engaged to her new boyfriend, he doesn't want to believe it," the source said.
"He is still broken hearted over their split and he can't imagine how she could move on so quickly, or fall in love already. It hurts Mac to think Ariana is considering marrying someone she barely knows. Mac loved Ariana for years, and it is like a punch in the gut to him that she would agree to marry someone new, after only dating them for a few weeks, and so soon after breaking up with him. Mac is crushed," the source added.
Grande and Miller, who had been dating for two years, ended their relationship this May.
Over the past one month, the couple attended each others' shows and Davidson also got two tattoos inked, including one of Grande's initials on his right thumb.
A source close to the new couple said Grande and Davidson were telling people that they got engaged past weekend at actor Robert Pattinson's house party.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)