SendBeacon does not fire on page close

I'm doing the following with javascript.

// Chrome (desktop) - refresh, navigate away
      // Chrome (mobile) - background mode
      // Safari (desktop) - change tab
      // Safari (mobile) - background mode
      // Firefox (desktop) - refresh, navigate away, change tab, close tab
      // Firefox (mobile) - background mode
      document.onvisibilitychange = function () {
        if (document.visibilityState == "hidden") {
          sendBeacon()
        }
      }

      // Chrome (desktop) - change tab, close tab, close browser
      // Safari (desktop) - refresh, close tab, close browser
      // Firefox (desktop) - close browser
      window.onbeforeunload = function () {
        sendBeacon()
      }

      window.addEventListener("beforeunload", function () {
        sendBeacon()
      })

      window.addEventListener("unload", function () {
        sendBeacon()
      })

      // Chrome (mobile) - refresh, navigate away
      // Safari (desktop) - navigate away
      // Safari (mobile) - refresh, navigate away
      // Firefox (mobile) - refresh, navigate away
      window.onpagehide = function () {
        sendBeacon()
      }

 const sendBeacon = useCallback(() => {
    const url = `/api/groupChat/logout`

    let headers = {
      type: "application/json",
    }

    let payload = {
      method: "publish",
      params: {
        channel: `server:${store.server.id}`,
        data: {
          origin: "centrifuge",
          event: "onlineState",
          data: { online: false, chatUserId: store.user.id },
        },
      },
    }

    let blob = new Blob([JSON.stringify(payload)], headers)
    navigator.sendBeacon(url, blob)
  }, [store.server.id, store.user.id])

Expected behavior:
Safari emits a beacon when the page closes

Orion and macOS:
Mac: Mojave 10.14.4
Orion: Version 0.99.109.1-beta (WebKit 613.1.12)