why the Video Agent would suddenly change the avatar and voice i

We successfully generated approximately 20 videos using the /v1/video_agent/generate endpoint with consistent avatar and voice. Then without any changes to our code or prompt, subsequent videos started using completely different avatars and voices. We did not change the prompt or any parameters between the consistent videos and the inconsistent ones.
Can you explain why the Video Agent would suddenly change the avatar and voice it selects mid-batch? Is there a way to lock a specific avatar and voice in the Video Agent endpoint so it stays consistent across all videos? Or is there a session/context that resets after a certain number of videos?