STEPS TO REPRODUCE
1. Run KRDC on a laptop (or the Steam Deck).
2. (Optional) Connect it to a external display (that is larger than the built-in laptop screen). Set the external display as the only active screen.
3. Connect to a remote server. In my case, the remote server had a larger screen/resolution than the client machine.
4. Enter full screen mode. Adjust the zoom as needed in the top toolbar.
5. Disconnect the laptop from the external display. Now the client resolution has changed, and KRDC stays fullscreen.
6. (Optional?) Reconnect the laptop to the external display. Now the client resolution has changed back, and KRDC continues fullscreen.
7. Try accessing the KRDC toolbar.

OBSERVED RESULT
The KRDC toolbar is nowhere to be found. It doesn't matter if it is the collapsible toolbar or the sticky toolbar.

EXPECTED RESULT
The KRDC toolbar should always be available, even if the client resolution changes.
What's worse: without the toolbar, there is no way (AFAIK) to un-grab the keyboard and to go back to windowed mode. The only solution I found was to use the Super key to invoke the KDE taskbar and then close KRDC.

SOFTWARE/OS VERSIONS
SteamOS (which in itself is based on Arch Linux)
Linux/KDE Plasma: 
KDE Plasma Version: 5.27.10
KDE Frameworks Version: 5.115.0
Qt Version: 5.15.12