Why does the sort order in my thread views keep resetting?
Plain saves your sort order in your browser, not on the server. When you change the sort in any thread view, such as the Done view, the preference is stored in that browser's local storage for your user and workspace. It does not sync to other browsers or devices, and it resets when local storage is cleared.
The default sort for every thread view is Status changed at, oldest first. You see the default the first time you open a view in a new browser, or after local storage is cleared, for example in a private or incognito window.
If your sort keeps reverting to oldest first, the most likely causes are using more than one browser or device, a browser extension that clears local storage, or opening Plain in a private window.
How to keep your preferred sort
Set the sort once in each browser you use. It persists in that browser until local storage is cleared.
For a sort that survives a fresh session, set the sort you want and bookmark the URL. The active sort is included in the URL as a query parameter, so the bookmark always opens the view in that order.
There is no workspace-level or user-level default sort setting, so you cannot change the default for everyone at once.