EDIT: i “solved” the problem in the way as follows:
In my custom AtomicTransactionComposer class, i add a condition on ABI method parsed return type and simply istantiate a new String from abiEncoded return value:
ABIType methodRetType = this.methodMap.get(i).returns.parsedType;
if(methodRetType instanceof TypeString) {
decoded = new String(abiEncoded);
} else {
decoded = methodRetType.decode(abiEncoded);
}