Once your tenant is using UPS (unified presence service), you should no longer have issues where DND changes back to available.
If you are on UPS, and are experiencing this issue, it would be super helpful if you can capture logs by typing Ctrl + Shift + Alt + 1, and send the file from your downloads folder along with any details to email@example.com.
An error occurred while saving the commentEdwin commented
Same. Still happening to me as of today. This is ridiculously stupid. Busy and DND status should never change on its own.