Rxjava: Insert only works with single but not with completable

So I have two DAO functions for inserting a new user:

@Insert(onConflict = OnConflictStrategy.REPLACE)
Completable addData(User user);

@Insert(onConflict = OnConflictStrategy.REPLACE)
Single<Long> insert(User user);

The difference is that one function returns a completable and the other one a Single<Long> where Long equals the id of the new inserted user.

Corresponding ViewModel functions:

public Completable addData(User modelClass) {
    return Completable.fromAction(() -> appDatabase.userDao().addData(modelClass))
            .subscribeOn(Schedulers.io())
            .observeOn(AndroidSchedulers.mainThread());
}

public Single<Long> addData2(User user) {
    return appDatabase.userDao().insert(user)
            .subscribeOn(Schedulers.io())
            .observeOn(AndroidSchedulers.mainThread());
}

Last but not least the subscribe method in my android activity:

viewModel.addData(new User( ""+myEntireName.getText().toString(), "jlk"))
.subscribe(
() -> { 
 Toast.makeText(this, "Jähgermeister!!!!!", Toast.LENGTH_LONG).show();
}, 
throwable -> {
Toast.makeText(this, "Error!" + throwable.getMessage(), Toast.LENGTH_SHORT).show();
});

viewModel.addData2(new User( ""+myEntireName.getText().toString(), "jlk"))
.subscribe(
newI -> { //onSucess
Toast.makeText(this, "Yeah i work: " + newI, Toast.LENGTH_LONG).show();
}, //onError
throwable -> {
Toast.makeText(this, "Error!" + throwable.getMessage(), Toast.LENGTH_SHORT).show();
});

So here is the issue: The Insert function which returns a Completable prints the Toast message but the user is not actually insertet. But if I use the function which returns a Single<Long> the User is inserted. Same goes for delete.

What can be the cause? Can Insert only work with Single<Long>?