Recent test builds seem to have implemented a function that automatically cleans up the inbox after a certain delay (not immediately). It seems to violate the principle of least astonishment in that it often happens when I’m inspecting or even editing a task I’ve already assigned to a project or context. I have “Clean up changed items” set to “When changing views.”
This was an unintended side effect of a fix for a different issue; test builds should include a fix for this soon if they don’t already. Sorry for the trouble it caused!