SUMMARY
When switching between keyboard layouts using a keyboard shortcut from the "main" hotkey list, no OSD appears. However, when I switch keyboard layouts using a user-defined "alternative" shortcut, the OSD appears.

STEPS TO REPRODUCE
1. Set up two or more keyboard layouts.
2. Pick a shortcut from the list for the "main" shortcut (here: Alt+CapsLock), and enter another shortcut as "alternative" shortcut (here: my keyboard's "Favorites" button) - see attached screenshot.
3. Switch keyboard layouts using the main, then the alternative shortcuts and compare. 

OBSERVED RESULT
The OSD appears only when switching using the alternative shortcut.

EXPECTED RESULT
The OSD appears only when switching using both shortcuts.

SOFTWARE/OS VERSIONS
Operating System: openSUSE Tumbleweed 20200625
KDE Plasma Version: 5.19.2
KDE Frameworks Version: 5.71.0
Qt Version: 5.15.0
Kernel Version: 5.7.5-1-default
OS Type: 64-bit

ADDITIONAL INFORMATION
Tested only on X. 
There are some other bugs that could be related, such as bug 408928, but there it's not clear which shortcuts people use ("main" and "alternative"), as a result of which the behaviour might not seem reproducible.