The two-time world drivers' champion qualified ninth and had been on for a possible top 10 on his return to F1 but was forced out with a braking issue after 33 laps of 56.
It has now been revealed that the drama was that foreign material had become lodged in a rear brake duct, which caused overheating to the brakes in Car #14.
“Both cars made a good start and gained some places on the first lap with Fernando holding firm in the top 10,” recounted Alpine F1 Team executive director Marcin Budkowski.
“After his first stop we had a small issue that forced us to reduce the performance of the car.
“Then after the second stop, a sandwich wrap paper got stuck inside the rear brake duct of Fernando's car, which led to high temperatures and caused some damage to the brake system, so we retired him for safety reasons.
“It was a very unlucky first race for Fernando considering how strong he looked.”
Despite the disappointment of a DNF, Alonso was at least pleased to be racing in F1 again, and in the thick of battle.
“Firstly, it was great to be back racing in Formula 1,” said the Spaniard.
“The start was fun, we gained some places and I had some enjoyable battles with old colleagues.
“However, it was disappointing to not see the chequered flag in the end.
“The issue we had after the second stop was a rear brake issue as some debris entered the brake ducts and overheated the temperatures of the car.
“Looking at the race it was very close in the midfield, and just a couple of tenths seems to change the order quite significantly.
“I think it's going to be a very interesting and competitive season to watch. We'll go again and fight hard in Imola [Round 2].”
Alpine left Round 1 emptyhanded after Esteban Ocon finished the grand prix, albeit in only 13th position.
The Frenchman qualified 16th, at least in part due to an unfortunately timed yellow flag, and his cause was not helped during the race when a hit from Sebastian Vettel on Lap 45 caused both to rotate.
“It wasn't the result we wanted, but there were certainly lots of positives,” said Ocon.
“We were fighting for most of the race with a number of cars with fun, wheel-to-wheel battles.
“Even with Seb [Vettel] at the end, he locked up and went into me. He apologised, we all make mistakes, and that happens sometimes.
“We were just a little bit short overall today. Qualifying didn't help us as it meant we always had places to gain. We showed our fighting spirit during this race and that's quite important.
“We move onto the next race as a team, aiming to get back to scoring points.”
Imola hosts Round 2, the Emilia Romagna Grand Prix, on April 16-18.