The "Auto-add" function in LrMobile is intended to automatically import any NEW images added to the Camera Roll from the point in time that the function was activated, i.e. it is not expected at the time of the "auto-add" activation that any existing images in the Camera Roll would be automatically imported into LrMobile.
The potential problem, however, is the ongoing use of iCloud and the optimise storage option. My understanding is that any images that were in the Camera Roll at the time of "auto-add" activation in LrM, and which were thus not automatically imported into LrM, could subsequently be replaced by lower res versions in the Camera Roll by iCloud....and I would expect that LrM would detect those new low-res versions as "new" images, and so would auto-import them. It's for that reason that I personally keep iCloud for Photos disabled on all my iDevices (and I recommend that all Lightroom Mobile users do the same).