Maybe because I used a plastic holder rather than wooden?
The film space is the same in the oldest "film only" wood holders and the most recently manufactured plastic ones. I think people who make this work in a double dark slide are taping the Instax print to the septum and then lightly forcing the dark slide into place over it. How they do the taping in the dark and get the print straight, I couldn't say... It might work better in a holder for glass plates, without the film sheath -- but then you'll have a focus offset.
; unfortunately youre the only other person online who had the idea to test it. I have a 2x3 speed graphic with a graflex back and finding a film holder for it is seemingly becoming impossible; I have literally searched every single sheet film holder listing on ebay for one and was hoping that instax would work with the grafmatic because those are at least available in some places. If you ever find the time to give it another test I would super appreciate an update 
