Intermittent 503 OriginTimeout when previewing Excel files with Office Online Viewer (view.officeapps.live.com)

We embed Excel files stored in Azure Blob Storage in our React application using the public Office Online Viewer:

https://view.officeapps.live.com/op/embed.aspx?src=<URL-encoded Azure Blob SAS URL>

The preview works intermittently for the same Excel file and the same page. Sometimes the workbook opens normally. Other times, the embedded frame displays:

Office Online services aren't available right now We're working to restore all services as soon as possible.

When this happens, Chrome DevTools → Network → All / Doc shows that the initial embed.aspx request is followed by a request to an Excel service endpoint similar to:

POST https://sg4-excel.officeapps.live.com/x/_layouts/xlembed.aspx?...
Status: 503 Service Unavailable
Content-Type: text/html
X-Azure-Externalerror: 0x80072ee2,OriginTimeout

The response body is the Office Online error page shown inside the iframe. Reloading the page several times can make the same file open successfully, without changing the file or application code.

We would like to understand:

1. Does 0x80072ee2,OriginTimeout mean that the Office Online Excel service timed out while retrieving the file from Azure Blob Storage, or could it indicate an internal timeout in the Office Online service?
2. What requirements should we verify for a Blob SAS URL used with this public viewer (for example, URL validity period, GET/HEAD access, response headers, redirects, or download latency)?
3. Is there a supported way for the embedding page to detect this specific failure? The failing xlembed.aspx request occurs inside Microsoft's cross-origin iframe. An iframe load event does not reliably indicate that the workbook rendered successfully.
4. Is there a recommended retry or fallback strategy for intermittent failures of the public op/embed.aspx?src= viewer?

We can provide a HAR capture, the complete response headers, the error page's Ref A/B/C values, and the UTC timestamp through a private support channel if needed. We do not want to post a live SAS URL publicly.

Environment: React application; Excel .xlsx files in Azure Blob Storage; public Office Online embed viewer. The issue is intermittent and has been observed with the same file.