Dear @adam
I have recreated your setup and the same occurs with me.
I make a full width image within Desktop Layout - Mobile Gridder options are set to 0% for the left and right edge frame.
When i go to change Gridder setup to Mobile Layout it is fullscreen on the backend but on the front end has the left and right margin as you say
but when i go and re-size the image in the Gridder again to the same size now it works fullscreen.
However when i go into Customiser i notice interestingly enough that it has its own set Mobile left and right frame percentage:
Screen Shot 2020-11-12 at 1.31.47 PM.png
So i also set this to 0% and now when i make a fullwidth image and change to Mobile Layout i keep the full width that we want.
Seems to be an issue with the Customiser having the last say on how the Mobile Layout should be!
Let me know if this works for you too and have a wonderful day
Best
Richard