It seems that in material-ui 5.0.0-beta which you're using it has problem. In 4.12.3 it's working properly. Please check out this codesandbox:

const ComboBox = function () {
  const [value, setValue] = useState("");
  const Combo = useRef();

  const onBlur = (e) => {
    Combo.current.inputValue = value;
  };

  const filterOptions = (options, state) => {
    return options;
  };

  return (
    <Autocomplete
      ref={Combo}
      onChange={(e) => {
        setValue(e.target.value);
      }}
      onSelect={(e) => {
        setValue(e.target.value);
      }}
      filterOptions={filterOptions}
      id="combo-box-demo"
      options={top100Films}
      getOptionLabel={(option) => option.title}
      style={{ width: 300 }}
      onBlur={onBlur}
      clearOnBlur={false}
      inputValue={value}
      renderInput={(params) => (
        <TextField {...params} label="Combo box" variant="outlined" />
      )}
    />
  );
};

I tried to set the input value manually but because of the material-ui version, It doesn't work.

Try using the latest version of the material UI, It's always good to use the latest version.

Try this sandbox with the latest version of MUI.

This is adapted from the documentation demo sandbox from here.