Hi Nik
I am using the Canon 7D Mark II.
I am seeing this problem when I am viewing JPEG files, not the RAW files
I created a new folder with just one JPEG file in it from this camera and I saw the same issue. If I look at the same JPEG file with RAW Power in my Photos library, the Z key zooms to 100% as expected so this only seems to be an issue with folder browsing.