It just happened to be that I accidentally double clicked on a file in the SaveAs dialog, which resulted in the file being overwritten without warning. I attended to mark the file instead in order to copy its name to use it as a pattern for the file I wanted to save.
I can reproduce the bug. Other KDE apps such as Spectacle and KolourPaint work as expected and will warn if the file will be overwritten (e.g. by double-clicking the file).
*** This bug has been marked as a duplicate of bug 420528 ***