It's reproducible, but I don't think it's a valid bug. Inert is defined in the HTML spec: https://html.spec.whatwg.org/#inert, and I suppose you could argue that since the iframe can't be focused so the keys shouldn't have any effect. However the keyboard multimedia keys are a user agent feature that works a bit independently, for example if there are two videos on the page both are stopped. As such I think they're no affected by inertness.
Bug 1845864 Comment 3 Edit History
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It's reproducible, but I don't think it's a valid bug. Inert is defined in the HTML spec: https://html.spec.whatwg.org/#inert, and I suppose you could argue that since the iframe can't be focused the keys shouldn't have any effect. However the keyboard multimedia keys are a user agent feature that works a bit independently, for example if there are two videos on the page both are stopped. As such I think they're not affected by inertness.