"Um.... engagement ring??????????????" the comment reads.
Allen responded with a joke referencing Brad Pitt's famous line in 1999's Fight Club, when his character says, "The first rule of Fight Club is: You do not talk about Fight Club."
Allen wrote back, "first rule of engagement club .........."
ET has reached out to Allen and Harbour's reps for comment.
This isn't the first time the couple has sparked rumors that they've taken their relationship to the next level. In March, the Stranger Things star referred to Allen as his "wife" during an Instagram Live session. When a fan asked Allen how many copies she sold of her album, No Shame -- which dropped in June 2018 -- she laughed, guessing, "I don't know, like 5,000?"
"You think you married a pop star? You didn't," she continued, referring to Harbour. "Not that we're married. We're not married. I just want to make sure you know."
But Harbour added fuel to the marriage rumors, interjecting, "But she is my wife, my wife," as Allen followed up with, "We do pretend."