SUMMARY

Global menu disappears when a window is maximised (1), or in the following cases:
2) a click outside a non-maximised windows (e.g. single click on desktop having dolphin in foreground)
3) a click inside a windows (even non-maximised) (e.g. open a new tab in google-chrome, or a folder in dolphin)


STEPS TO REPRODUCE
For case (1)
1. Open dolphin (as non-maximised window)
2. Maximise
3. The global menu disappears and does not come back
For case (2) and (3)
1. Open any application
2. Click on the desktop or click on some objects within the open application
3. The global menu disappears and does not come back

OBSERVED RESULT
The global menu disappears and does not come back unless the windows lose and gain the focus again. Anyway, the global menu disappears again as soon as one of the previous actions is performed.

EXPECTED RESULT
The global menu should be always present for the foreground application.


SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Archlinux
(available in About System)
KDE Plasma Version: 5.15.0
KDE Frameworks Version: 5.55.0
Qt Version: 5.12.1

ADDITIONAL INFORMATION
I suspect that these commits broke something (more likely the first one):
https://phabricator.kde.org/D16768
https://phabricator.kde.org/D17107

The bugs doesn't appear on plasma-workspace 5.14.5 (even rolling back after the upgrade to plasma 5.15).