I had this problem again (original poster). Rather than do a full reset of the Apple TV, I found this page that describes how to reboot the remote:
https://support.apple.com/en-gb/HT203777
Followed the steps and it fixed the volume issue. Again.
At least it's quicker and easier than a full reset. Hope this helps someone else experiencing the issue.
sarah15
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Reseting to factory defaults and reprogramming the remote fixed the issue. The issue is resolved.