Wikipedia declares: The monarchy of the House of Savoy formally ended on June 13, 1946, and Umberto left the country. https://en.wikipedia.org/wiki/Umberto_II_of_Italy
Did King Umberto II abdicate?
"...In hopes of influencing public opinion ahead of a referendum on the continuation of the monarchy, Victor Emmanuel formally abdicated in favour of Umberto on 9 May 1946 and left for Egypt.Before departing for Egypt, Victor Emmanuel saw Umberto for the last time, saying farewell in a cold, emotionless way....."
"..In the 2 June 1946 referendum, a 52% majority voted to make Italy a republic. The conservative, rural Mezzogiorno (southern Italy) region voted solidly for the monarchy while the more urbanised and industrialised Nord (northern Italy) voted equally firmly for a republic. In northern Italy, which had been ruled by the Italian Social Republic, the charges of homosexuality made against Umberto had an impact on the voters, causing at least some conservatives to vote for the republic. From his exile in Egypt, where King Farouk had welcomed him as a guest, Victor Emmanuel expressed no surprise at the result of the referendum as he always viewed Umberto as a failure who was unfit to be king, and claimed that the monarchists would have won the referendum if only he had not abdicated. Umberto himself had expected to win the referendum and was deeply shocked when the majority of his subjects chose a republic…"
"..The republic was formally proclaimed four days later, ending Umberto's brief 34-day reign as king. Umberto at first refused to accept what he called "the outrageous illegality" of the referendum, and took his deposition badly. In his last statement as king, Umberto refused to accept the republic, saying he was the victim of a coup d'état by his ministers and the referendum had been rigged against him…."
"..Some monarchists advocated using force to prevent a republic from being proclaimed, even at the risk of a civil war, but Mack Smith wrote that: "Common sense and patriotism saved Umberto from accepting such counsel".Umberto rejected the advice that he should go to Naples, proclaim a rival government with the intention of starting a civil war in which the Army would presumably side with the House of Savoy under the grounds that "My house united Italy. It will not divide it".The monarchy of the House of Savoy formally ended on 12 June 1946, and Umberto left the country. Prime Minister Alcide de Gasperi assumed office as Italy's interim Head of State. At about 3 pm on 13 June 1946, Umberto left the Quirinal Palace for the last time with the servants all assembled in the courtyard to see him off and many were in tears. At the Ciampino Airport in Rome, as Umberto boarded the air plane that was to take him to Lisbon, a carabiniere grabbed him by the hand and said: "Your Majesty, we will never forget you!”...."
So I wonder if he really officially abdicated, he (just) left Italy to go in exile after the pro-republic outcome of the referendum