I stored a running Konsole in my Plasma session, including three open Konsole tabs. (I hope the wording makes sense... I used "Save Session" in the start menu to let Konsole - besides other applications - be auto-started upon login.) My current Plasma version is 5.12.4. Since yesterdays upgrade from KDE Applications 17.12.3 to 18.04.0 and Frameworks 5.44.0 to 5.45.0, Konsole does not show a transparent background any more by default. If I enter the settings, it states that Konsole was started before compositing was enabled. As it's expected, the transparent background works if I quite and restart Konsole, but I lose my default tabs in this case... Transparent background worked fine before yesterday's update, even when Konsole was started by the session management.
The following bug apparently is unrelated, but one of the comments there points to an earlier, similar problem: https://bugs.kde.org/show_bug.cgi?id=366368#c6
*** This bug has been marked as a duplicate of bug 362857 ***