To me, it appears as if the two-finger scrolling signal doesn't properly get picked up by the VirtualBox host software, and/or doesn't properly get passed on to the guest.
I tried multiple emulation types in the VirtualBox settings (PS/2 mouse, tablet, multi-touch tablet), tried reinstalling the add-ons in the Guest OS, but nothing helps. When you are using your computer and want to scroll the screen with your mouse to view your files, but you find that your middle mouse button is not working, which is very disappointing.
I tried debugging it using xinput and evtest in the guest: indeed, the guest OS only receives scroll signals when I very aggressively use the two-finger scroll on my touchpad. When I scroll very aggressively, then some gesture gets picked up by the guest OS, and the page suddenly scrolls a lot. Two-finger scrolling with touchpad almost never works.Scrolling with my normal mouse works fine in my VirtualBox guest as well (Ubuntu 19.04).Scrolling with both normal mouse and touchpad (two-finger scrolling, Dell Precision 5520) works fine on my VirtualBox host (Ubuntu 19.04).
