I recently installed the full KDE package on a LinuxMint 19 installation on my PC, but when I tried to use the Dolphin program I saw that the standard title bar was missing. Who do I need to contact about this issue?

Which desktop environment are you running when you tried to run dolphin, Cinnamon or Plasma? (what happens if you log out of the one desktop and log in to the other?)
Could you post a screenshot which demonstrates the problem?