Keeping the avatar look the same in all videos

We have an app in which we create avatars & videos using Heygen

First step, we use the Heygen Avatar creator API to create the Avatar. This trains the avatar & an avatar group is created
Next step, we use the Heygen Video creation API & to generate videos using that avatar. Our app sends the content via the api and the avatar in Heygen is used

The trouble is that if we create many videos using that avatar group, we FEEL that with every video, the avatar starts lookimg lesser and lesser like first avatar.

How can we prevent this? How can we make sure that for each subsequent video, the avatar looks the same