This conversation has been merged. Please reference the main conversation: Photoshop CC 2019: With rulers show, brushstroke creates a straight line as if Shift was held down (macOS)
The issue is resolved only by turning off the ruler.
I have experienced it on MacPro 2013 running 10.14 Mojave and on brand new MacBook Pro 16, 10.15.2 Catalina, only running latest Photoshop 21.0.2 and latest Wacom Driver 6.3.38-2. Nothing else yet installed.
The problem doesn't exist on MacPro 2013 running 10.12.6 Sierra.
Also testes with Wacom Intuos 4 XL (PTK-1240) and Intuos Pro M (PTH-651)
I'm aware that there were some issues with the ruler, but thought that thew are resolved in the last version. Adobe please, this is a serious issue. It is not possible to work like this.