Why does sub_filter seem to not work when used in conjunction with proxy_pass?

Solution 1:

Your response probably has other content-type than defined in sub_filter_types by default.

Reference: http://nginx.org/r/sub_filter_types

Solution 2:

James T Snell answered it in a comment:

I didn't have proxy_set_header Accept-Encoding ""; you need that to tell the backend that compression isn't allowed in a response.