Is your LR configured in a language other than English? If so, that's a known bug. See the comment made by employee Simon Chen underneath this post (you have to click View More Comments to actually see it): Lightroom 6: Negative Values Bug (German). The comment includes a workaround that involves editing a text file TranslatedStrings.txt.
Also, add your vote and opinion to that thread, which is in the official Adobe feedback forum. The more votes and opinions, the more likely Adobe will prioritize the fix.
Thank you for reacting on my question.
I use Lightroom 6 in Dutch.
I now altered it in English and the problem is vanished.
Hope this annoying bug will be fixed in nearby future.
If using it in English becomes too inconvenient for you, you could try the workaround discussed in the other thread of editing TranslatedStrings.txt.