Some progress. I read somewhere (I don't remember the page) that someone got it to work properly by updating the intel graphics driver. So I updated my Intel Xe Graphics driver to the latest version from Intel (not Dell), and the scrolling seems to be working smoothly now. At least so far. I'll see if it regresses again (hopefully not).
Like many others who have had issues with touchpad two-finger scrolling not working reliably on my XPS 13, I spent a significant amount of time trying to find a solution and finally found it. Download and install the Dell driver below and your problem should be solved (don't worry about the compatible systems): [URL]
Touchpad two-finger scrolling solution