Duplicants lose their faces as of the latest version.

It happened while chatting after eating and peeing on breaking time for me.

I'm going to take a random stab and say this is related to something in the patch notes about a lot of content with this update not being in yet, though it's one that is... unnerving in a way.

9 hours ago, Ipsquiggle said:

Please note: The socialization and recreation systems are still being roughed in, so you'll find more loose ends and missing details than usual at the moment. We still have to add things like tooltips and icons, UI for the schedule, attributes and effects actually modifying each other, and so forth. We encourage you to comment on the game as you're currently experiencing it, as we'll be considering your feedback while we develop these systems.

It's doing this every time during break when the dupes should be having the new facial actions.

edit: that I think on it, the faces can disappear any time they start talking to each other, breaks or other times.

Edited July 13, 2018 by MagicManICT

With the latest update, this bug still occurs, though not in as many situations.  In particular, their mouths will not appear during break time conversations... until they have the "Took a Break" / "Talked with Friends" QoL buffs.  (I'm not sure which of the two it is.)  Once those buffs appear, their mouths come back and are restored for any further conversations during that break time.