Text highlight color jumps to black

This sounds a bit weird, but when I assign a highlight color to phrases in my text, it happens every so often that a text search results in turning the searched-for text bit automatically into black highlighted text. I can remove this by the undo command oder by selecting the phrase and manually change it back to ‹no color›. But the issue keeps coming back.
Thanks for suggestions.
highlight-background-colorhighlight-background-color2