I think it is not a persistent disabled, maybe a upgrade or create a new account will re-enable it. You know, Settings app is usually aimed at current user rather than global system.
Please also disable the audio device from Device Manager and registry. Refer to this article below:
Disable a Sound Output Device in Windows
https://www.tenforums.com/tutorials/128343-enable-disable-sound-output-device-windows.html
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