SUMMARY
When you're using vector layers, and you change the cap and join styles for a line, the next line you draw won't have the same cap and join styles, and there is no way to change the default cap and join styles for vector lines.

STEPS TO REPRODUCE
1. On a vector layer, draw a bezier curve with corners.
2. Select it with the "Select Tool", and change the cap and join styles in the Tool Options.
3. On the same vector layer, draw another bezier curve with corners.

OBSERVED RESULT
This bezier curve doesn't have the cap and join styles you used at step 2.

EXPECTED RESULT
This bezier curve should have the cap and join styles you last used.

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION