In my headcanon the Stormtroopers from Cloud City allowed the heroes to escape, because Vader had ordered the sabotage of the Falcon’s hyperdrive and there was the change Luke might run away from Vader and to try to escape from the city. With Luke’s X-Wing probably taken down or heavily guarded, Luke’s only hope would be to telepathically call Leia. That way, Vader would capture all the good guys and Lando would reveal himself as a Rebel sympathiser, thus granting the Empire a plausible justification for taking Cloud City.

If Vader did not let Leia, Lando and Chewbacca escape, Luke would have remained in Cloud City until he could get a safe way to leave the city, and the Imperials would never capture him again (maybe Vader would have tried to torture Leia, who knows). If the Falcon is outside Cloud City, Luke’s first choice would be that ship.

Given how smart Vader is in TESB, I think it makes sense. But it’s headcanon.