When I try to use the stylus pen over a scrollbar in the plasmoids (for example on Stick note or KDE app menu) is imposible to drag the scrollbar. It can be reproduced in Plasma 5.12.5, and Plasma 5.13. The problem affects some scrollbars besides the plasmoids (in the desktop effects stack for example). All softwars as Krita, dolphin, kmail, etc, work fine. Here I recorded my Screen https://youtu.be/yWslqgDhZ0o My Wacom is a CTH480 Intuos Pen and Touch. I tried with KDE Neon 5.13 and Opensuse Leap 15 same results. In Opensuse 43.2 with plasma 5.8 LTS all works perfectly.
Can confirm this erratic behavior being present on my system with the same Wacom model as OP. 2 more users with different hardware in the KDE Neon Telegram support group said to be affected too: https://t.me/kdeneon/55101
Please report this issue directly to Qt developers via https://bugreports.qt.io/
Looks like this old bug: https://bugreports.qt.io/browse/QTBUG-56481 I think Plasma uses org.kde.plasma.extras.ScrollArea which is based on ScrollView