James has had trouble maintaining a poker face before, most famously in 2018 after J.R. Smith tried to dribble out the clock in a tied Game 1 of the NBA Finals. ESPN says that James fessed up to the problem after several teammates called him out on it during a meeting back in February.
Via the story on the Lakers' lost season:
"The meeting in Memphis was a retort of sorts. In what became an open forum, several players spoke up. Players focused on James’ inconsistent body language throughout the year. The four-time MVP copped to the critique, telling his teammates that, in essence, cutting out behavior like slumped shoulders and sideways glances has been something he has tried to work on his entire career."
The players-only meeting was organized by Rajon Rondo after James very publicly aired out his team. James questioned the Lakers drive to get to the playoffs and laid the blame at the fact that many of the team's members have never been to the postseason.
"How many know what's at stake if you've never been there?" he asked, at the time.
Silver Screen and Roll editor Harrison Faigen pointed out that LeBron still slips, noting his frustration at Kyle Kuzma after he failed to complete a cut to the inside.
Old habits die hard and LeBron doesn't make a habit of losing.