A highly resented White House appointment and the deportation of a stowaway have unexpectedly rankled the anti-Castro Cubans of Miami, who are counted on to lead the way in Hispanic electoral support for President Reagan.

The Miami area's 750,000 Latinos will largely remain enthusiastic Reagan supporters, their leaders say. Foreign policy is the key to how most of Miami's Hispanic voters make up their minds, they add, and Latinos here generally consider Democratic candidates soft on Cuba.

Hispanics here nevertheless are muttering about what they regard as administration bungling on two issues that touch them directly: the naming of Carlos Diaz Alejandro to the presidential commission on Central America and last year's expulsion of a refugee who is said to have ended up in a Havana jail.

The first problem began when the White House announced that a commission headed by former secretary of state Henry A. Kissinger would investigate Central America's turmoil and suggest long-term U.S. policy there. On the membership list was Diaz Alejandro, a Cuban-American economist at Yale University.

He was picked in an evident effort to represent and please Cuban-Americans, who under the Reagan administration have been unusually active in Washington and have provided several cheering backdrops for policy pronouncements on Central America.

Instead, his name set off alarms in Miami and inspired protests to the White House. Those who selected him apparently were never told that many Cuban-Americans here, including those working most closely with the administration, regard him as dangerously pro-Castro.

"We really question his background to be on a commission that will have to have access to classified information," said Jorge Mas Canosa of the Cuban-American Foundation, which had Reagan here for a warm Little Havana reception in May.

Diaz Alejandro was one of the Committee of 75, Cuban-Americans who in 1978 helped arrange family visits to Cuba by refugees living in the United States. Many Cubans here regard any contact with Castro's government as next to treason.

Diaz Alejandro also has been associated with Areito, a magazine published in the United States that often supports Castro's government. Moreover, he helped sponsor a letter to President Carter five years ago urging moves toward normalization of relations with Cuba.

Ciro del Castillo, of the Congress of Dissident Intellectuals, said Miami Cubans also have been upset by reports that a Cuban who stowed away on a boat to Miami and then was deported last year apparently has been jailed in Cuba.

The refugee, Andres Rodriguez Hernandez, was the only Cuban to flee the island and then be shipped back since Fidel Castro came to power in 1959, according to the Immigration and Naturalization Service. The INS deported him under steamlined procedures for stowaways and according to the 1980 Refugee Act, ending a practice of granting nearly automatic asylum to Cubans.

Another 40 Cubans who arrived more recently are being held in Krome Detention Center near here pending resolution of similar asylum requests. Some could be deported, although that is unlikely in light of a legal standoff over U.S. efforts to deport 1,500 undesirable refugees from the 1980 Mariel boatlift who are now imprisoned in Atlanta.

Rodriguez' deportation has become a symbol of concern, not only for the Krome and Mariel detainees but also for Cubans who have exit permits but are still awaiting U.S. visas, delayed by the standoff.