Try using a lower Feather setting and/or Heal mode. If that doesn't work please post a Fit view screenshot of the image with the Spot Removal tool showing.
The bright spot looks like you used the brush tool with + exposure. When you clone the area with the spot tool it will also take on the brush adjustment. Turn on the brush tool and see if there is an adjustment there and if so delete it.
Thank you for your suggestion.
The problem persists even with less feathering. It seems as though the "brightness" of the pale timber affects the resulting clone (image 1), despite setting opacity to 100. To test that I sampled from the darker foliage into the pale area and it does appear that even the foliage turns paler at the destination (image 2), and even more so when cloned onto the blemish itself (image 3) though this is less marked.
The result is about the same when healing.
So far these behaviours are all happening to raw (.NEF) files. What is most odd is that the tool works fine when I edit a JPG file exported from the raw (image 4).
Image 1 -- editing a raw (.NEF) file
Image 4 works correctly on a JPG
Thanks. Adjustment Brush tool was not set. And setting it at minus did not seem to affect this either .
(Note FWIW that the blemish is due to a drop of rain on the glass.)
Bob Somrak is correct. You have an Adjustment Brush or Radial Filter applied to the water spot. Open the Adjustment Brush and click on Reset at the bottom its panel. Next open the Radial Filter tool and click on its Reset. Now try the Spot Removal tool again.
What appears to be happening is that the Adjustment Brush and Radial Filter adjustments are applied AFTER the Spot Removal tool application. The cloned area has the Adjustment Brush and Radial Filter Applied on TOP instead to the original area BELOW.
I discovered a similar issue with the Transform panel application as described here:
Thank you, that is spot on!
What made me think it was a water artifact was that it recurred in the same place in nearby shots. But that was just due to my sync-ing develop settings.