SUMMARY

STEPS TO REPRODUCE
1. Open properties of some WireGuard VPN interface;
2. On the right pane in the default 'WireGuard Interface" tab click the Peers button;
3. In the "WireGuard peers properties" dialog you should see at least one "Peer %n" tab;
4. Click to focus on any of the text fields inside the tab's frame;
5. Press Escape key on the keyboard.

OBSERVED RESULT
Tab's content is gone. It becomes hidden or something. If you have more than one Peer tab, switching between the tabs back and forth fixes the visibility.

EXPECTED RESULT
not this

SOFTWARE/OS VERSIONS
Operating System: Arch Linux 
KDE Plasma Version: 6.3.5
KDE Frameworks Version: 6.14.0
Qt Version: 6.9.1
Kernel Version: 6.14.10-arch1-1 (64-bit)
Graphics Platform: X11

ADDITIONAL INFORMATION
This is probably some artifact from Qt Widgets architecture wherein the distinction between parented widgets and top-level widgets/windows is quite intangible at times. So…

FOR ANYONE TRYING TO FIX THIS: make sure the tab's content isn't exposed as some kind of top-level widget elsewhere, and if it does — make sure it can still close itself with the parented Escape-to-hide behavior removed.