Okay, found the issue! A change I made to the GUI system to make the new character design interface work has accidentally broken the quest interface for non-active quests.
(technical details follow; mostly just me talking to myself)
The issue is “MultiViews”; a GUI container which can display one of several different “pages” inside a part of the GUI. The “Info” windows, for example, are almost entirely composed of a single big “MultiView”, which contains a bunch of different pages, and the tabs along the bottom of the window change which “page” is being displayed, when you click on them.
The specific change I made was to only set the “page” when the MultiView becomes visible, instead of setting it immediately when the MultiView is created. (This was necessary for technical reasons, to support GUI elements which contain 3D scenes. I won’t go into why it was necessary, because I don’t want to cry.)
The ‘Quest’ panels in the NPC info window are actually another MultiView; they contain three pages each; the “Quest Info” page, the “Add a quest” page, and a “Blank” page. When the NPC Info window opens, each panel is initialised to display the right correct page. But because of that change above, when the panels come into view, they all get reset back to the “Quest Info” page whenever they come into view.
The fix has been to make MultiViews remember when something has told them to switch to showing a different page, and set whichever page they were most recently shown to display, instead of showing their default starting page, when they come into view.
That fix will be coming in the next build, probably later today.