I really don't see any reason for South Tyrol not to rejoin Austria.
For one, I don't think that Austria will be prepared to give them the same generous benefits as Italy does. Also, South Tyrol has a substantial Italian and Ladin, which would be strongly against South Tyrol joining Austria, which could cause substantial problems. Note that the Italians are actually a majority in Bozen, the capital of South Tyrol and which is quite close to the southern border of the province. The South Tyrolians might find themselves without their capital if they attempt this...
Around a quarter of the South Tyrolean population are Italian, most live in Bozen/Bolzano which is a linguistic enclave, only a single municipality (Salorno) with Italian majority are connected to the rest of Italy. If the southern Tyrolean "isthmus" stayed Italian even with Bozen it would be 50%+ German. Also because of most of the Italian population in South Tyrol are urban settlers placed in the province under Fascism, if the economic fundament for Bolzano disappear (the German speaking hinterland and Bolzano's position as the centre of administration) at lot may migrate afterward to the rest of Italy, risking that Germans end up in majority in the areas which would stay Italian.
Ladins on the other hand make up 5% of the population and and have absolut majority in connected 8 municipalities (and make up the majority in the eastern part of a German majority municipality), which lies connected to the rest of Italy. So theorectic they could easily be cut off from the rest of South Tyrol and stay Italian if there are a majority supporting it. Which are not given.