CDNs (mostly Azure Front Door) is using HTTP Range Requests to retrieve data from Origin when caching enabled.

Express should ignore Range: requests header when gzip compression is in place since Express is unable to respond a HTTP/206 compliant answer with the right computed Content-Range header in the response that is taking the compressed data length into account.

A fair compliant workaround is to, in that very case of compression where computing Content-Range values would be too complex, ignore client's Range: header in the request, and answer the whole compressed content in a HTTP/200 response.

Handling Range: headers is optional so answering a HTTP/200 is OK.

Answering a HTTP/206 with wrong Content-Range values is notOK.

Meanwhile another workaround is to disable compression and make CDN handle it, or disable CDN caching, however it would be fair to expect Express to return a compliant HTTP response in any case.

References:

RFC7233

Details and highlighting Express behavior:

https://github.com/DanielLarsenNZ/nodejs-express-range-headers