SUMMARY
In the Okular search field (Ctrl+F) you can press Enter to go to the next match and Shift+Enter to go to the previous match. Pressing the enter key on the numpad goes to the next match but pressing Shift+numpad Enter still goes to the next match and not to the previous.

STEPS TO REPRODUCE
1. Open search bar on a .pdf document and enter a search string
2. Press Shift+Enter ("Return") to get to the previous match
3. Press Shift+numpad enter ("KP_Enter") and observe that Okular jumps to the next match instead of the previous


SOFTWARE/OS VERSIONS
KDE Plasma Version: 5.17.4
KDE Frameworks Version: 5.65.0
Qt Version: 5.14.0

ADDITIONAL INFORMATION
I've just checked in KMail in the mail view search field and funnily enough there the behavior is the other way around, numpad enter doesn't work but shift+numpad enter works.
Also I've read through the Okular source code a bit and it doesn't seem like the bug is caused by Okular because it uses the returnPressed signal from KLineEdit (which is QLineEdit?, didn't check that) and then checks "QApplication::keyboardModifiers() == Qt::ShiftModifier" and not much more. Still, it's a bug that's present in Okular :)
Also maybe https://cgit.kde.org/falkon.git/commit/?id=a237c8ff874a2adeed8588709f14fdb3b766fa2f is useful