It would appear that this ends up being tracked under the cgroup for the unit daemon, resulting in this problem when a restart of the unit daemon occurs; as this can happen across all unit daemons in the pxc cluster during a juju upgrade-juju operation, this is probably the root cause.
However, I would expect the follower units to have been started normally, and not be part of the unit cgroup - so only the lead unit should see this effect.
The charm has to init the local instance using:
service mysql bootstrap-pxc
It would appear that this ends up being tracked under the cgroup for the unit daemon, resulting in this problem when a restart of the unit daemon occurs; as this can happen across all unit daemons in the pxc cluster during a juju upgrade-juju operation, this is probably the root cause.
However, I would expect the follower units to have been started normally, and not be part of the unit cgroup - so only the lead unit should see this effect.