When implementing Interactive Avatar using the Heygen Streaming API, the video becomes a still image.

I'm currently testing embedding Interactive Avatar on a website using the Heygen Streaming API.
https://docs.heygen.com/docs/streaming-api-integration-with-livekit-v2
Based on this document, I'm testing the HTML & Javascript implementation shown under "Here's a full working HTML & JS implementation...", but

Voice conversation is possible.(OK)
The avatar becomes a still image.(NG)

I contacted support and was given the following checklist to ensure the call was successful:

Verify:

LiveKit room connection is established
STREAM_READY
event fires and media stream is attached (
videoElement.srcObject = event.detail
)
videoElement.play()
is called
You're sending tasks via
/v1/streaming.task
endpoint

After checking, I found that videoElement.play() had not been called, so I changed it to calling mediaElement.play() immediately after

mediaElement.srcObject = mediaStream;

However, the result remained the same and the avatar remained static.
I suspected a problem with the network I was testing, so I disconnected the VPN and tested again, but the result remained the same.
Are there any other possible causes?