Tom
The only place I see this bug occurring is in the site footer although I haven’t tried putting 4 columns of contained text into the body of the site or anywhere else. If I did I suspect it would happen there too.
A fresh install like you’ve used in your video example typically populates that footer widget content from WordPress, not from directly within the site itself. If you go into the test sites footer widgets you will see those fields are actually empty. Also if you use the WordPress default content, it typically behave in a fluid fashion and not like contained content does.
Try adding some text content into four separate footer columns and see what happens. One word per line using paragraphs or line breaks.
They should behave differently and instead of being like they are in your example with 3 widgets sitting on top of each other at 767px they should instead become two by two like generatepress.com does at 768px having two columns on top side by side and two side by side directly below that. This way you should get a 2 x 2 footer widget layout, not a 1 x 3 configuration as shown above in your video.
Also if you check your site the 2 x 2 footer widget configuration happens at 768px and at 767px you get 1 x 4 columns sitting on top of each other. This is what should happen.
I suspect that will help you replicate the 767px pixel grid bug, providing you understand what I mean now.
Cheers