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Miley Cyrus can buy herself flowers, but she might not have to ... 'cause she has fans convinced she's engaged to Maxx Morando .

The "Wrecking Ball" hitmaker had fans chatting after arriving at the "Avatar: Fire and Ash" world premiere in Los Angeles on Monday with a diamond ring on that finger. Getty

The sparkler -- set in a thick gold band -- couldn't be missed as she rested her left hand on the musician's chest as they posed on the red carpet. Getty

Fans are also sharing a photo they believe Maxx's father shared online, offering his congratulations to the longtime couple ... and the ring can be seen there as well.

TMZ has reached out to reps on both sides ... so far, no word back.

Miley and Maxx were first linked when they attended the Gucci Love Parade togethe

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