Testing Consumer.plainSource never sees produced messages

I made a post on stack overflow here that will be easier to read than posting in here. I have a short description of the error and my question below.

My current code uses a Consumer plain source to connect to a websocket streaming messages produced to a kafka topic to some end client. When I run this code locally I can successfully consume from the topic and send it back to a websocket connection. However trying to test this using containers frequently gives me errors related to the source timing out waiting for the next message.

Is my current test setup correctly? I can successfully read the messages using the TestKits consume method so producing to the topic is not the issue. Thanks in advance