LookerEmbedSDK '/auth' endpoint gives CORS error when other requests to same endpoint don't

[RobinShift4]

I'm trying to use the LookerEmbedSDK to embed dashboards.

LookerEmbedSDK.init(LOOKER_USE_EMBED_DOMAIN,
        {
          url: `MYURL/auth`,
          headers: [
            {name:'authorization', value:`Bearer ${getStorageToken()}`},
            {name: "Content-Type", value:"text/xml"},
          ],
          withCredentials: true
        })
const lookerEmbedSDKResponse = await LookerEmbedSDK
        .createDashboardWithId(currentDashboard.dashboardId)
        .appendTo(el)
        .withParams({_theme: "{\"show_filters_bar\":false}"})
        .withFilters(currentFilters)
        .build()
        .connect()
        .catch((error) => {
          console.error("An unexpected error occurred", error);
        });

But I consistently get

Uncaught (in promise) TypeError: Cannot read properties of undefined (reading 'send')

Which is occurring in embed.ts:253

In my network tab I see this is a CORS error (I also see that I have properly set the Authorization token)

The thing is - other requests to that same endpoint (via Curl or axios) work and do not give me an error, they give me what I expect.  What's going on with the Embed auth call logic?