I am sorry but I couldn’t reproduce your issue. Did you add d+ as a regular expression at the bottom of the list of highlight strings in the Highlight (1) tab of C++ configuration properties?
Yes, I added it last with only Regular Expression ticked.
I had added it with color number 8 to make it easy to spot, and now that I try again I can see it only does it when I select one of the colors with a background color in it (1, 8, 9 or 10).