PHP prevent header overwriting by Proxy - apache

Question

I want to access a PHP script hosted on dnsserver.icu via this proxy 207.154.231.211:8080 with curl.
The problem is that the Proxy server seems to do overwrite the HTTP 200 code with a 302 code making it impossible to reach the script.
curl -v dnsserver.icu gives the following output:
* Rebuilt URL to: dnsserver.icu/
* Trying 134.122.73.150...
* TCP_NODELAY set
* Connected to dnsserver.icu (134.122.73.150) port 80 (#0)
> GET / HTTP/1.1
> Host: dnsserver.icu
> User-Agent: curl/7.58.0
> Accept: */*
>
< HTTP/1.1 200 OK
< Date: Wed, 15 Apr 2020 20:05:18 GMT
< Server: Apache/2.4.29 (Ubuntu)
< Content-Length: 31
< Content-Type: text/html; charset=UTF-8
<
* Connection #0 to host dnsserver.icu left intact
whereas curl -v -x 207.154.231.211:8080 dnsserver.icu gives the unexpected result of:
* Rebuilt URL to: dnsserver.icu/
* Trying 207.154.231.211...
* TCP_NODELAY set
* Connected to 207.154.231.211 (207.154.231.211) port 8080 (#0)
> GET http://dnsserver.icu/ HTTP/1.1
> Host: dnsserver.icu
> User-Agent: curl/7.58.0
> Accept: */*
> Proxy-Connection: Keep-Alive
>
< HTTP/1.1 302 Found
< Location: http://206.189.153.135
< Date: Wed, 15 Apr 2020 20:08:37 GMT
< Connection: keep-alive
< Transfer-Encoding: chunked
<
* Connection #0 to host 207.154.231.211 left intact
The address in the Location header is also changing sometimes.
I already experimented with different header configurations but I couldn't get it to work. When I log every call to the PHP script it doesn't look like the server is even reached by the proxy (no call logged). Futhermore the apache access log is empty.
Strangely this is not the case for all domains. I'm able to access e.g. ip-api.com, google.com or also less popular domains like proxyjudge.us (but not the ip equivalent of 45.33.35.141) through the proxy.
I have no idea what the reason for this behaviour is. Is there any 'trick' in terms of header setting or apache configuration that makes it possible to also access dnsserver.icu through this proxy? Something I havent tought of?
I appreciate any help.