Photo Mechanic displays the embedded JPEG in your RAW files when viewing the RAW file on its own (and not combined into a single thumbnail like your last screenshot, in which case it prefers the JPEG).
Fuji has historically embedded a pretty small preview in their RAW files, with every camera up until recently embedding a preview with a resolution of 1920x1080. I'm not sure when they started embedding larger previews, but I believe it was within the last year or two. It isn't impossible that RAF files contain another larger JPEG preview, which would indeed require us to push an update, but considering that most if not all Fuji cameras have embedded a preview smaller than the full resolution of the image, I'm guessing there isn't a larger preview than what you are seeing.
Feel free to post a sample file.