Thanks, I found an Adobe case with almost the exact same RTX 40-series issue.
https://community.adobe.com/questions-729/cant-export-in-hardware-encoding-1557056
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Please check Control Panel >> Programs and Features and uninstall any WebM plugin/application. If you don’t see WebM, search for fnord instead, then restart Premiere.
Adobe confirmed this plugin can block NVENC hardware encoding even when CUDA is working normally.