Lens Correction Vignette disabled

[bst7]

Hi,

Using the Lens Correction tab, Develop module of LR 6.x, the "Vignette" slider on the first pane of that tab is always disabled. I've tried various lens profiles on the "Leica M" camera, and none have the Vignetting slider enabled (see below). If I set the camera make to "Nikon", then the Vignetting slider becomes active, so this seems to be something deliberate/unique to the Leica M model. Does anyone know why? I guess this is something about the way the Lens Profiles for these cameras are constructed...?

William

Here is how the pane's "Profile" tab appears (consistent regardless of Lens I use)

It is enabled in the "Manual" tab:

[tazztone]

i have a similar situation with the Lumix S5.

i disabled vignetting correction in my S5 coz i read it could mess up the image quality in vlog. unfortunately it also disabled it for RAW photos. now in Lightroom the "vignetting" slider is greyed out under "lens corrections". anyone knows how to "force" apply the Lens profile? there is only "camera settings" there. and slider greyed out
https://imgur.com/a/rrYH93h 
cheers