This is fairly reproducible. If you have an existing combat encounter in a map with multiple required target squads, and you decide to remove units from the objective, you can lose all of the dialog and mission-text in the preceding map-change dialog box.
Basically, if you delete the squad units from the map first (resulting in them becoming "unplaced"), and then try to re-select the squads in the combat objective in the story-pane after deleting the wrong squad from the objective, the whole storyline will "twitch" and result in the dialog being reset back to the defaults.
I got hit by this twice in the same mission because I didn't think to rename the squad units, and the names were long enough that I couldn't see the whole name to choose which to delete from the objective. So I thought I'd just re-select the squads and make sure they were all correct... and then noticed my map-transition text was all gone.