Their red-carpet smiles didn't last long.
As Miley Cyrus and Liam Hemsworth posed at the LA premiere of Avengers: Endgame on April 22, the newlyweds' wildly different styles were on full display – and the tension between them was palpable.
"Miley wouldn't stop talking about how she wanted to let her hair down and have a crazy night out," a source reveals.
But Liam, there to support his brother Chris who stars as Thor, was "hoping to have a fairly chilled evening".
And neither was happy by the end of the night.
As the Wrecking Ball singer vamped it up for photographers, her Aussie actor hubby grew increasingly uncomfortable, resulting in the famous duo getting into their first major fight as a married couple.
"After they walked the carpet, Liam angrily accused her of turning the premiere into the Miley Show!" says the source.
"Once they got home, the bickering blew up into a big, fat argument."
Disagreements are nothing new in this relationship. It was only after a 10-year on-off romance – which included a broken engagement in 2013 – that Miley and Liam finally tied the knot, pulling off a surprise wedding in December.
Still, "Liam had hoped Miley would grow up after marriage," dishes the source.
"He thought they might even have a baby on the way by now. But Miley's just not ready to settle down like that."
WATCH: Miley Cyrus and Liam Hemsworth tie the knot. Story continues after video...
Indeed, the pressure points that drove them apart in the past seem to remain a problem.
The 26-year-old songstress, who posted several Instagram shots celebrating the unofficial marijuana holiday on April 20, "is still smoking pot and partying until the early hours of the morning", the source says.
"She continues to act like a teenager, and Liam has reached his boiling point."
As tensions simmered the night of the premiere, "Miley told Liam to stop being overbearing," shares the source.
"She even threatened to stay at her mum's for a few days, so they could have breathing room."
But the Isn't It Romantic actor, 29, wouldn't have it.
"Liam persuaded her to stay," adds the source.
"He told Miley he loves her, and that's all she really needed to hear."
Both are discovering that married life is not as easy as they thought.
"But Miley and Liam are crazy about each other,' says the source, "and they're committed to making things work".