I have an X1 Carbon 6th Generation (Core i7 8th Gen, 16 GB RAM, 512 GB SSD) and it behaves like described in the first comments above.
Specifically, when I close the notebook it somewhat goes in standby (or not really, I'm not sure), the red dot blinks but underneath the notebook gets hot within a few minutes. And sometimes, not always, it freezes (or stays frozen) instead of waking up when I open the notebook again.
I have an X1 Carbon 6th Generation (Core i7 8th Gen, 16 GB RAM, 512 GB SSD) and it behaves like described in the first comments above.
Specifically, when I close the notebook it somewhat goes in standby (or not really, I'm not sure), the red dot blinks but underneath the notebook gets hot within a few minutes. And sometimes, not always, it freezes (or stays frozen) instead of waking up when I open the notebook again.
I'm running 18.04 Bionic (development branch).