Ah, yup, there it is! subprocess.Popen() doesn’t invoke a shell, it takes your arguments array and passes them to the target program as-is. This means that quotes and * characters alike are untouched, so using **/footage/** without quotes here is safe.
And, of course, using quotes means they get passed to restic, which will dutifully ignore any paths that start and end with a " character, and have /footage/ somewhere in them. 
Thanks for satisfying my curiosity.
Oh and look, you aren’t the first person to trip up here. I knew this sounded familiar… I helped someone else out with this exact issue!