Thanks for the posts.
Please be aware that if you are using a third party audio product such as a soundbar, the reason you may be seeing this behavior is that the Roku TV may be trying to match the volume level/control of the soundbar.
For example, if the Roku TV volume control is from 1-100 and soundbar volume control is from 1-50, changing the volume control of the Roku TV would not correlate with a 1:1 change with soundbar volume.
We hope this provides clarification regarding the issue you are experiencing with your Roku TV.
Roku Community Moderator