> Is this expected behaviour? Could there be another way to do this?

'if' (the rewrite module) is executed in early stages when $sent_* variables are not available that’s why the regex doesn't match.

What you could do is use 'map' instead:
(http://nginx.org/en/docs/http/ngx_http_map_module.html )


map $sent_http_link $IS_WORDPRESS {
~*.*wp-json.* "true";
default "false";
}

And then:

add_header X-Is-WordPress $IS_WORDPRESS;

rr

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