Unnecessarily strong type constraints with polymorphic variants (when using `React.useState`?)

The following works (ReScript Playground):

let f = v => {
  let x = ref(#b)
  x := v
}
let g = () => {
  let x = #a
  switch x {
  | #a => f(x)
  }
}

but the equivalent code using React.useState does not (ReScript Playground):

let f = v => {
  let (_x, setX) = React.useState(() => #b)
  setX(_ => v)
}
let g = () => {
  let (x, _setX) = React.useState(() => #a)
  switch x {
  | #a => f(x)  /* This has type: [#a]
  But this function argument is expecting: [> #b]
  The first variant type does not allow tag(s) #b */
  }
}

I can explicitly annotate x: [> #a | #b] in g, but it makes the switch non-exhaustive, and moreover, it is not a general solution as x can no longer be applied with other functions accepting different poly variants, e.g., [> #c].

What is causing this problem? How can I bypass this?