The 10-year-old Irish-bred horse collapsed after narrowly beating Thistlecrack to win the Cotswold Chase at Cheltenham's Festival Trials Day.
He was treated by veterinary staff before being pronounced dead.
The British Horseracing Authority confirmed the horse suffered from a "severe pulmonary haemorrhage".
Tony Welsh, acting chief veterinary officer for the BHA, said: "Episodes such as this are rare, and can occur in horses which have no underlying health issues, and amongst all disciplines of sport horses.
"In spite of the rarity of these incidents, as a sport we are determined to do more to understand what causes these symptoms, and whether more can be done to prevent it."
In Saturday's race, King George winner Thistlecrack caught Many Clouds close to the finish but the Trevor Hemmings-owned gelding, ridden by Leighton Aspell, fought back to win.
Many Clouds had shown symptoms of post-race ataxia in the past, which had seen him over-heat and "wobble" on his feet, but the horse had never collapsed before, and had shown no symptoms of post-race ataxia on Saturday.
Many Clouds had previously won the Gold Cup trial in 2015 and the Hennessy Gold Cup in 2014.
Trainer Oliver Sherwood described him as the "horse of a lifetime".
BBC horse racing correspondent Cornelius Lysaght
The crucial words here are "no significant underlying health issues".
On more than one occasion, notably after his Grand National success, Many Clouds received treatment for a tendency to 'overheat', but he soon recovered, and his welfare was closely monitored by the authorities as a result.
With that fact in mind, questions have been raised as to whether that condition was linked to his death, but this report concludes he was the victim of a haemorrhage rarely seen on the racetrack.
A post mortem examination revealed Many Clouds died from bleeding on the lung but that the 2015 Grand National winner had no underlying health problems.