UseSWR shows undefined data when trying to update fallbackData whereas useState does not

[ridhwaans]

import useSWR from 'swr'

export const useSharedState = (key, initial) => {
  const { data: state, mutate: setState } = useSWR(key, {
    fallbackData: initial,
  })

  return [state, setState]
}

I'm using the above SWR hook as a replacement for useState and an object state (SWR API options). However partially updating the fallback data/initial data or mutating at least one of the fields doesnt work as expected. The playerState.repeat key is undefined although it is defined initially

import { useSharedState } from "../hooks/useSharedState"

const REPEAT_STATES = Object.freeze({
    REPEAT_OFF: 0,
    REPEAT_ONE: 1,
    REPEAT_ALL: 2
  });

const [playerState, setPlayerState] = useSharedState('player',
    {
      currentIndex: null,
      playlist: [],
      isPlaying: false,
      currentTime: 0,
      duration: 0,
      isMuted: false,
      repeat: REPEAT_STATES.REPEAT_OFF,
      shuffle: false
    }
  );


const toggleRepeat = () => {
    if (playerState.repeat == REPEAT_STATES.REPEAT_OFF) {
      setPlayerState(playerState => ({
        ...playerState, repeat: REPEAT_STATES.REPEAT_ONE
      }));
    } else if (playerState.repeat == REPEAT_STATES.REPEAT_ONE) {
      setPlayerState(playerState => ({
        ...playerState, repeat: REPEAT_STATES.REPEAT_ALL
      }));
    } else if (playerState.repeat == REPEAT_STATES.REPEAT_ALL) {
      setPlayerState(playerState => ({
        ...playerState, repeat: REPEAT_STATES.REPEAT_OFF
      }));
  }
  console.log(`toggleRepeat state: ${playerState.repeat}`)
}

> toggleRepeat(); // toggleRepeat state: undefined

However this issue doesnt occur when I switch to the useState hook; calling toggleRepeat() returns/mutates the initial value

const [playerState, setPlayerState] = useState({
      currentIndex: null,
      playlist: [],
      isPlaying: false,
      currentTime: 0,
      duration: 0,
      isMuted: false,
      repeat: REPEAT_STATES.REPEAT_OFF,
      shuffle: false
    })
...
> toggleRepeat(); // toggleRepeat state: 0

Is that intended, or am I misunderstanding how SWR manages state?