List View, Start with a Rich text document.
When I use Data>Convert>to Plain text, Kind changes the RTF to Plain text document.
But when, using the same Plain text document, I use keyboard Cmd+A (select all) followed by Shift+Cmd+T, then click OK in the dialogue box, Kind doesn’t change to Rich Text Document until i click on another document.
The first command creates a new, converted item. The second one changes the format of the document you’re currently editing, therefore the kind changes after (auto-)saving the document.
And also note this is the same behavior found in Apple’s TextEdit - the Shift-Command-T converting the document in-place. DEVONthink uses the same text engine, as provided by Apple, so features generally work the same in both apps.