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Similar thing happened to me with my Apple TV 4K. When I was watching it, Bluetooth devices in the vicinity (including its remote!) became choppy and unreliable. Turned out to be the fault of the HDMI cable! Replaced it and the problems all disappeared.

MacOS might occasionally be weird, but that definitely sounds like some kind of electrical issue, probably nothing to do with the OS.

Possibly... But I don't understand why that particular HDMI cable was causing issues with my USB devices -- with no visual artifacts or latency or other issues being evident on my screen.

But you're right -- I was possibly too quick to snark about MacOS when the real culprit is more likely the cheap random HDMI cable I used. :)

Most likely the HDMI cable has significant signal leakage (poor or missing shielding), and the signal has some component (or harmonic) in the same spectrum as your dongle.