Clone stamp shows 'negative' of sampled area inside the tool

Only recently I have noticed that the clone stamp tool's behaviour seems to have changed, because the cloned area now appears to display a negative or inverted colour version of the sampled area, whereas it just used to show the sampled pixels. Choose an opacity of, say, 30% and, as the stamp is applied by left-clicking, it clones pixels from the sample area OK but, as the cloning builds up, the interior of the tool gradually darkens, so you can't really see what you're doing.

In the example below I've got a tool of 175px with a hardness of 30% and I Alt-clicked the target on the house's doorway. The screen shot shows the tool's appearance, which looks like a negative of the sampled area.

Is this behaving correctly, or has there been a change to Photoshop, or do I perhaps have a graphics driver problem?

Any assistance would be greatly appreciated, thanks.