SUMMARY
There is a disconnect between scrolling with the mousewheel and the page-wise scrolling by clicking at the page control area of the scrollbar. E.g, if scrolled down a few pages by clicking at the page control area and then using the mouse wheel, the page jumps back where it was before clicking at the page control area. If the slider is used, then everything is fine. 

STEPS TO REPRODUCE
1. Move the slider to the top of the document
2. Click below the slider to scroll a few pages down
3. Using the mousewheel now, the document jumps back to the top

OBSERVED RESULT
The mousewheel ignores the current position but uses the position before scolling down some pages.

EXPECTED RESULT
The mousewheel should continue to scroll from the current position.

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: openSUSE Leap 15.2
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION
I also compiled the latest version (1.11.70) but the problem is still there.