If you set the focus prevention on level high or extreme the launcher doesn't appear if there is any window with focus. Reproducible: Always
Similarly with the calendar on the digital clock.
Can confirm this bug for both App Launcher and Calendar in: Distribution: KDE neon Developer Edition Unstable Branches KDE Plasma Version: 5.10.90 KDE Frameworks Version: 5.37.0 Qt Version: 5.9.1 Kernel Version: 4.10.0-32-generic
*** This bug has been marked as a duplicate of bug 352647 ***