I had used the feature to use embedded previews since it became available. My Fuji x-trans files were able to be viewed 1:1 without problems, just as my Canon 5D files - I saw no difference in handling these in the Library Module. Only since the latest LR update has the problem I described occurred. I'll check later, if my Canon files show that same strange performance.
This paragraph, from the link you provided, might explain the phenomenon I've encountered, though I can't completely understand what it means:
"Embedded preview is a camera preview, which is created by the camera, and is available with the raw image. Latest Nikon and Canon cameras have full size embedded previews. Means, the camera-generated embedded previews can be used for culling. Sony, Olympus, and Fuji cameras don’t have full sized embedded previews. In this case, Lightroom checks if the embedded preview size is 50% of that of the raw image. If not, the standard preview is created instead of reading the embedded preview. Create a jpg sidecar image along with the raw. If the image has the sidecar file, it’s being read for creating the preview in the Library module."