I was unable to capture a screen of the bug as it occurred due to it’s nature, and even if I had I am unable to upload it because I’m a “new user”, so hopefully my description will be accurate enough. My settlement was almost entirely underground via being dug into the side of a mountain, among other design problems this entailed (I would love to be able to put doors in underground without having to dig enough space to put up walls) I encountered an odd bug at dinner time.
Sometimes if I didn’t watch them all go into the dining hall a few would end up vanishing until they had finished eating, I would then witness each of the disappeared Hearthlings suddenly “fall” from above the ceiling onto the dining table closest to the entryway to the dining area.
Welcome to the forum, @Whooda. 
This bug has been reported some time ago. Which version were you playing?
I’m suspecting that this teleportation has to do with the fact that they don’t have enough height space on caves when they are using the chairs, but I might be wrong (this also happened in normal dining buildings).
Thanks for reporting and for the details you provided 
Thank you @Relyss I was playing in Alpha 10, the latest patch version as of mid-June. Oddly enough I have not seen it happen again since I posted this.
I agree on the possible cause though, it seemed to always happen right when they would normally be getting up from their seat, with a ceiling only 4 blocks high. It could be some sort of clipping issue between the table, chair, and low ceiling where they don’t have enough space to animate properly. So instead the game displaces their model and when the trigger happens for standing it just plops them back where it thinks they should be.
Just a guess though, despite my vague technical knowledge I’m certainly not a programmer