In August 2017, right in the middle of production on Mission: Impossible 6, Cruise mislanded a rooftop jump in London, breaking his ankle. The stunt was meant to have Ethan Hunt grabbing onto the edge of the building, but the actor impacted incorrectly, smashing into it. Filming was put on hold for, with Cruise returning in October for the final weeks of shotting. Footage of the jump was shared at the end of 2017 and subsequently appear in trailers for Mission: Impossible - Fallout. It looked like Cruise, ever a stickler for authenticity, was allowing the messed up take - which he finished due to in-the-moment budget concerns - to be used in the film. Not so!
The sequence in question comes at the end of Mission: Impossible - Fallout's second end, where Ethan is chasing an assailant through the City of London from St. Paul's Cathedral to the Tate Modern on the Southbank. And while the much-discussed jump does appear, the take is different: Cruise lands with his weight on the edge and his foot doesn't unnaturally bend. After being in the marketing, the accident wasn't used after all.
However, it does appear that the aftermath of the injury did make it into the film. When Ethan pulls himself up from the edge and continues his pursuit, he limps for a second. Comparing this to the footage of the accident, it appears this is the take of Cruise diligently finishing the stunt.
At the moment, neither Cruise nor writer/director Christopher McQuarrie has commented on the cut, although the reason is easy enough to deduce: it didn't look right. This may be a pretty simple stunt alongside the insanity of Mission: Impossible, but they still to make it look genuine. Cruise's landing in the failed attempt was off (hence the break) so it won't have necessarily fit into the wider action sequence. This is why the subsequent limp could be used - it looks fine in camera as the action (walking out of shot) is a lot less complicated.
As for why it was in the trailers, this is hardly the first time a movie has marketed itself on footage not in the film - and there's little about Mission: Impossible - Fallout that's captured people's imagination more than Cruise's injury (except perhaps Henry Cavill's mustache).