[lxc-users] ps -eo lxc no longer works

[Forest]

When I use the ps command's "lxc" format specifier, for example:

ps -eo pid,lxc,command

The second output column is supposed to show "the name of the lxc container
within which a task is running.  If a process is not running inside a
container, a dash ('-') will be shown."  This worked fine until I upgraded
from ubuntu 19.04 to 19.10, which brought me from lxc 3.03 to 3.04.  Now,
that column always contains a dash instead of the container name, even for
processes that are running inside a container.

Can someone explain why this broke, and how to fix it?

My containers are unprivileged, in case that matters.