Byte-range request ignored

I'm supporting this very old (circa 2013) app and byte-range requests aren't working on my static-only app.

This is the response from my static-files app:

$ curl -I 'http://static-url' -r -2           
HTTP/1.1 200 OK
Server: nginx
Date: Mon, 29 Apr 2019 17:30:14 GMT
Content-Type: video/mp4
Content-Length: 5633872
Connection: keep-alive
Last-Modified: Mon, 29 Apr 2019 17:18:37 GMT

And this is a response from another server I have

$ curl -I 'https://static-url' -r -2
HTTP/1.1 206 Partial Content
Server: nginx/1.10.3
Date: Mon, 29 Apr 2019 17:30:13 GMT
Content-Type: video/mp4
Content-Length: 2
Last-Modified: Tue, 07 Aug 2018 14:41:40 GMT
Connection: keep-alive
ETag: "5b69afa4-363a83"
Access-Control-Allow-Origin: *
Content-Range: bytes 3553921-3553922/3553923

According to this other question byte-range requests are supposed to be supported:

https://community.webfaction.com/questions/9424/how-can-i-enable-byte-range-request

The static-only app config page doesn't have any options to enable it.