It does seem to be related to joining citizens. Happened again in the new release. 15:29 in the video: information missing from dialogs. 15:50: selection information missing. Did not check for other signs of UI freezing (forgot at the time that you had pointed that out as a possibility), but there you go.
EDIT: Oh! The debugging tool thing in the bottom right was open, and you can see there that that is, in fact, frozen.