Just check if there is some local adjustment in place, that selected for the Subject or else the Background at the time it was made - and may still be reflecting the (former) presence of this hair, and causing a black appearance there.
Such masks do not automatically self-update when you in effect change the image content by cloning or whatever. They were generated at a certain moment in time, based on the photo as it then appeared. But you can later click Update to cause them to be re-generated based on the photo as it now appears. This should get rid of any "shadow" traces surviving from formerly-seen content such as stray hairs.
Or if it was a manually brushed Mask, that might now need some manual update. Either way, to clone out any unwanted content before applying masked adjustments, is generally speaking the more efficient work sequence.
Wow, I didn't expect that. Yes, there is an AI Mask for the Hair to make it darker and for the Skin to make it brighter. I used the Spot removal after those. But my experience in LR since Version 1 was, that LR allways "knew" what the right order of the adjustments is. So, for me it is still is a bug. Thank you both for your help!
LrC does "know" about the processing order for applying adjustments, both local and global, in relation to spot healing. Clone healing is not invalidated by changing your adjustments either. And mask types such as a gradient filter will just apply smoothly "over the top of" adjustments and clone healing whatever those may be.
The issue here, is when a mask derived from underlying image content (disregarding any adjustments) at a certain moment. The results from generating an AI mask are cached (a bitmap, AFAIK) for performance reasons - in a new folder alongside the Catalog - and not constantly re-generated on the fly.
It's similar with a manually made mask: say, that.you've carefully brushed to avoid / to follow 'something' within the photo. When that 'something' gets cloned out - your manually brushed mask will now need to be updated in the places where that 'something' used to be. Same for a PS layer mask that's been made from the luminance of the image at a certain moment: that too will get 'invalidated' by later cloning.
One nice thing about an AI mask, in comparison, is that there's an Update button!
It's not a bug! Adobe has stated numerous times that it's a limitation of the current design. Ideally, the user should be alerted that the mask(s) should be updated, but the request for same has not yet been implemented. However, it's also easy to fix - open the masks panel and 'Update' the mask(s) using the button provided (see attached screenshot).