Comment 41 for bug 1752165

I'm also experiencing this issue on a Thinkpad T480 (20L5) connecting to a ThinkPad Thunderbolt 3 Dock. It worked fine on Ubuntu 18.04 LTS until about a week ago, when I started experiencing this issue and the bug in https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1813663 more or less simultaneously.

The latter issue was resolved by downgrading to kernel 4.15.0-43, and later upgrading to 4.15.0-45.

This one remained a problem even after upgrading to Ubuntu 18.10 and kernel 4.18.0-14. I can confirm that installing kernel 4.20.6-042006 via Ukuu fixes it.