@ekkstatic Hi. Since you’re using “El Capitan” you must be using the legacy version of Pencil2D for macOSX. Unfortunately this problem has been reported plenty with Wacom Bamboo and CTL tablets as seen here Huge bug: Pencil won't work properly, creates tons of extra lines. PLEASE HELP!
The problem is that most Wacom tablets work fine (they don’t exhibit this issue), but it’s those two models specifically along with El Capitan (10.11) or Windows 7/10 that show this problem.
While it’s not entirely the hardware’s fault despite these tablets being refurbished (which is why they are cheaper than the other verisons), having an older macOSX version will limit your ability to get the latest updates and fixes available for your tablet which in the case of the Pen & Touch version their last update was from 2015
For now it doesn’t really seem like we can solve this directly at the moment, but can you try using the latest version of the software and see if this improves or worsens your experience? You can download it from the official website at http://pencil2d.org/download
Just don’t delete the version you’re currently using so you can have a base for comparing both versions (and you can have multiple Pencil2D version in your computer, since they are not installed but are rather portable)
The new version has a small issue with the layout so you probably need to follow the following advice (video included) to stack panels in order to avoid their contents getting weirdly displayed (devs are already looking for a fix) here’s a video with a simple workaround to avoid the panels crushing into each other: https://streamable.com/eqd1z0