macOS Dictation listening to internal audio?

I use the macOS dictation feature from time to time, so tend to keep it switched on (but not automatically listening).

I've noticed sometimes it is taking commands or random text when I play videos with spoken words in them. This can be YouTube, BBC, or others.

At first I thought it was taking sound from the speakers in to the microphone, but I've either turned the input volume right down, or selected "line in" as the input sound source (there is no line in sound source). If I enable Dictation manually (with these settings) it doesn't work (i.e. the input volume controller is working).

It's frustrating as I've had random apps open or bits of text inserted in apps. It happens maybe 2-3 times per day (depending on how much audio or video I'm playing).

I have a USB mixer hooked up and is usually set to "USB Play" mode. The sound is routed to my speakers via this so I wonder if the sound is being taken back into the Mac and then being processed. However, I've never selected the USB hub as an input sound source for system audio.

I use a Mac Mini so there is no internal microphone.

Anyone else come across this? I can workaround by switching Dictation off when I don't need it, but it's frustrating nonetheless.