After long disconnection from controller due to out of disk space, agent is removed.

[Seyeong Kim]

There was out of disk space issue for some days.

After recovered that, the customer found out that one of their etcd node's agent got lost.
/var/lib/juju/* ( agents, tools ) are removed.

In cmd/jujud/agent/machine.go
Runfunc (a *MachineAgent) Run(*cmd.Context) (err error) {
...
        err = a.runner.Wait()
        switch errors.Cause(err) {
        case jworker.ErrTerminateAgent:
                err = a.uninstallAgent()
...

uninstallAgent removes itself..

[Anastasia (anastasia-macmood)]

We have recently dealt with bugs in the area and the machine agent code lost this code, see https://github.com/juju/juju/blob/2.6/cmd/jujud/agent/machine.go

I believe that this scenario has been addressed by recent work. The fix will be available in the next Juju releases - 2.6.4, 2.5.8 and edge snap for future 2.7 stream.

[Anastasia (anastasia-macmood)]

I think most of the related work has been tacked through bug # 1827009. This report may very well be a duplicate.