This is not a defect, this behaviour is as designed
You can change from select -> multiselect because any associated data can be converted. The data for select field will convert to a single item chosen within the multiselect. No data would be lost, and so the conversion is allowed.However, the converse is not true - there would be no way to interpret the data of 2 chosen values back into the single select. Thus, allowing the conversion would potentially lose data, and so it is not allowed.The same is true of Number -> Text (because 3 can become "3"); but Text -> Number does not make sense because "abc" cannot have a numeric value.