Confluent SchemaRegistry Client ignoring system proxy settings

Question

trying to connect to Kafka but with Schema Registry behind proxy that requires authentication. I'm aware that Schema Registry Client right now does not support providing proxy credentials (https://github.com/confluentinc/schema-registry/issues/2137).

However I tried to set the proxy settings on system level and reuse them in my Java app, that connects to the Schema Registry and tries to put a simple message.

Attempt 1:

1. set http_proxy, https_proxy, HTTP_PROXY and HTTPS_PROXY system variables (using the http://<proxy_user>:<proxy_pass>@<proxy_host>:<proxy_port> syntax
2. verify connectivity via CURL - works!
3. run my Java application with system parameter:

java -Djava.net.useSystemProxies=true -jar ./avro-producer-2.1.0-jar-with-dependencies.jar

result: Error connection refused. Seems like the proxy parameters were ignored completely.

[2024-01-12 16:56:47,210] (io.confluent.kafka.schemaregistry.client.rest.RestService) - ERROR Failed to send HTTP request to endpoint: https://xyz.cloud/subjects/mycorp.crm.campaign. 
V1-value/versions 
java.net.ConnectException: Connection refused (Connection refused) 
        at java.net.PlainSocketImpl.socketConnect(Native Method) ~[?:1.8.0_352] 
        at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:350) ~[?:1.8.0_352] 
        at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:206) ~[?:1.8.0_352] 
        at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:188) ~[?:1.8.0_352] 
        at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392) ~[?:1.8.0_352] 
        at java.net.Socket.connect(Socket.java:607) ~[?:1.8.0_352] 
        at sun.security.ssl.SSLSocketImpl.connect(SSLSocketImpl.java:293) ~[?:1.8.0_352] 
        at sun.net.NetworkClient.doConnect(NetworkClient.java:175) ~[?:1.8.0_352] 
        at sun.net.www.http.HttpClient.openServer(HttpClient.java:463) ~[?:1.8.0_352] 
        at sun.net.www.http.HttpClient.openServer(HttpClient.java:558) ~[?:1.8.0_352] 
        at sun.net.www.protocol.https.HttpsClient.<init>(HttpsClient.java:264) ~[?:1.8.0_352] 
        at sun.net.www.protocol.https.HttpsClient.New(HttpsClient.java:367) ~[?:1.8.0_352] 
        at 

Attempt 2:

1. running my Java application with all proxy parameters in command line:

    java -Dhttp.proxyHost=myproxy.myorg.com -Dhttp.proxyPort=8080 -Dhttp.proxyUser=myuser_dev -Dhttp.proxyPassword=p3aFxxxx -Dhttps.proxyHost=myproxy.myorg.com -Dhttps.proxyPort=8080 -Dhttps.proxyUser=myuser_dev -Dhttps.proxyPassword=p3aFxxxx -Djdk.http.auth.tunneling.disabledSchemes="" -Djdk.http.auth.proxying.disabledSchemes="" -Djdk.https.auth.tunneling.disabledSchemes="" -Djdk.https.auth.proxying.disabledSchemes="" -jar ./avro-producer-2.1.0-jar-with-dependencies.jar

result: Error proxy returns authorization required

    (io.confluent.kafka.schemaregistry.client.rest.RestService) - ERROR Failed to send HTTP request to endpoint: https://xyz.cloud/subjects/mycorp.crm.campaign. 
V1-value/versions 
java.io.IOException: Unable to tunnel through proxy. Proxy returns "HTTP/1.1 407 authenticationrequired" 
        at sun.net.www.protocol.http.HttpURLConnection.doTunneling(HttpURLConnection.java:2154) ~[?:1.8.0_352] 
        at sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.connect(AbstractDelegateHttpsURLConnection.java:195) ~[?:1.8.0_352] 
        at sun.net.www.protocol.http.HttpURLConnection.getOutputStream0(HttpURLConnection.java:1342) ~[?:1.8.0_352] 
        at sun.net.www.protocol.http.HttpURLConnection.getOutputStream(HttpURLConnection.java:1317) ~[?:1.8.0_352] 
        at sun.net.www.protocol.https.HttpsURLConnectionImpl.getOutputStream(HttpsURLConnectionImpl.java:264) ~[?:1.8.0_352] 
        at io.confluent.kafka.schemaregistry.client.rest.RestService.sendHttpRequest(RestService.java:178) [avro-producer-2.1.0-jar-with-dependencies.jar:?] 

Java version is OpenJDK 1.8.0_352

What am I doing wrong?