I consider it a bug, but it might also be seen as a missing feature of an additional safeguard. This case has rather low repeatability, but might be painful to the user. I already faced it a few times.

The problem occurs when you edit the details of a task that is on a date-based list (for example, "today"). When you add a note (most time-consuming and most painful when losing), and when it's exactly past midnight while you're writing, the list reloads (for new today), the task you are working on is no longer a part of the opened list. Everything disappears, and you continue typing into nothingness.

There are warnings and questions if you want to discard your incomplete note when you try to change the task or list. I would expect a similar warning or any other blocker, preventing the user from losing content when the date is changed.