When konsole is started automatically after a desktop login and it previously was in full screen mode, the konsole permanently will lack window borders (unless you close and restart konsole).
STEPS TO REPRODUCE
1. start konsole and press F11 to change to full screen mode.
2. keep the konsole open but restart the desktop session.
3. select the automatically started konsole and press F11
The konsole window is not full screen but the window is borderless.
(available in About System)
KDE Plasma Version:
KDE Frameworks Version:
*** This bug has been marked as a duplicate of bug 403948 ***