tooltip not show when prop type is `union`

[flixyudh]

Current behavior

I just create my own React Native component, I have a component with mixing prop type like this:

fontSize: PropTypes.oneOfType([
  PropTypes.number,
  PropTypes.oneOf(["XXL", "XL", "L", "M", "S", "XS", "XXS"]),
]),

but there's no list of available type in union type.

To reproduce

you can see my styleguide.config.js, also the docs/ generated here

Expected behavior

I'd like to see something like @mitsuruog type declaration at https://github.com/styleguidist/react-styleguidist/issues/1621#issuecomment-653561591

so in my case, i expect to see "XXL" | "XL" | "L" | "M" | "S" | "XS" | "XXS" | number