I've never seen such a issue before.
Do you have tried the same after you have restart Lightroom and/or the whole computer?
Do you have tried to deactivte the GPU support?
Do you have tried t ouse the "Heal" option instread of "Clone"?
Restarted Lightroom twice, no change. I'll try the GPU thing when I get back to my comp
Also, I tried Clone, Heal, change Opacity, Feather, basically all the sliders in that tool. Only seems to affect that one spot on the photo.
My ******* had a random radial filter synced from another photo that appeared to be a spot lol. False alarm.