Comment 3 for bug 1767995

This affects me too. I noticed it as soon as I upgraded from 16.04 to 18.04.1.

It took me a while to find a workaround, so I'll document it here to help others. I believe there are other possible ways around this, but this one worked for me:

First, I ran synclient, to find the current scroll values. For me they were:
    VertScrollDelta = -73
    HorizScrollDelta = 73

As a test, I ran this command:
synclient HorizScrollDelta=-73

That fixed the problem, but only until the next reboot. To run this every time I started up, I created a script named FixNaturalHorizontalScroll which contained just that line, and I saved that script in /usr/local/bin. I used this command to make it executable:

sudo chmod +x /usr/local/bin/FixNaturalHorizontalScroll

Finally, I had to manually run gnome-session-properties in order to add my new script as a startup script.

I would be happy to help debug this. It's going to be a pretty frustrating introduction for new users coming from other systems. And it took me a few hours to piece together that workaround.