Nikon Z6, 24-70mm @ 70mm, f/4, 1/20sec handheld. ISO was not saved ('Hi' ISO = 64000 - 204800). Full frame, FX.
I purchased the new Z6, rather than the Z7, for better low light performance. Why the Hi ISO value was not saved in EXIF is a mystery*. Because extended ISO isn't 'real' ISO?
ISO values in the normal range, 100 - 51200, do get saved.
*Technical: I am talking about jpg files and looking at what Windows File Explorer displays under 'Properties/Details'. I have used Photoshop 'File Info' and find the same. I do not have 'ExifTool'. I know there are several other ISO tags, not shown, like ISOsetting and ISOinfo, but....