SUMMARY
When using a user-defined stylesheet, setting a different padding "destroys" the rest of the styling

STEPS TO REPRODUCE
1. Create a custom CSS file with the content 

QTabBar::tab { padding: 8px; }

2. Apply the stylesheet in Konsole preferences -> Tab Bar

OBSERVED RESULT
The tabs do respect the new padding but also lose a bunch of other styling information (e.g. the blue highlight...)

EXPECTED RESULT
The tabs should only change the padding, not the rest of the styling

Note: This does not apply generally to all settings in the custom CSS file. If you put

QTabBar::tab { min-width: 200px; }

then only the minimum width of the tabs is changed, the rest of the styling appears to be unchanged, e.g. the blue highlight line is still intact.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: KDE Neon / Kubuntu
KDE Plasma Version: 5.23.2 / 5.18.5
KDE Frameworks Version:  5.87.0 / 5.68.0
Qt Version: 5.15.3 / 5.12.8