Can't set custom status code

I've been having serious troubles setting the status code on returned pages. I don't know if I'm doing something wrong, there is a bug, or simply this is not supported.

I created this snippet by following some help from Google groups

rewrite_by_lua_block {
      ngx.status = 206
      ngx.req.set_uri("/index.html")
}

I would expect that this will forward /index.html with status code 206. Instead it returns 200.

My original snippet was like below (same behaviour)

content_by_lua_block {
      ngx.status = 206
      ngx.exec("/index.html")
}

I'm running the official Docker image: openresty/1.15.8.3. Following the documentation, this should work. From my digging I also found an old bug here but don't know if it's related.

In the end of the day, I simply want to return /index.html with a specific status code (depending on the status from the backend API).

Reproduce the behavior using the rewrite_by_lua_block and content_by_lua_block entry points shown in the issue with the openresty/1.15.8.3 image. Compare the documented ngx.status behavior with the internal redirects to /index.html and determine whether the status is expected to survive; done means the cause and supported behavior are established, with a regression test or documentation change identified.