If you have a timestamp filter that is set to end 12 AM for a day, it will actually give you all of the messages for that day. You can even see in the filter description in the bottom right that the time portion is removed.

But if I specify a non-midnight time, like 12:01 AM. It works as expected.

I also found that if the first timestamp box is a later date than the 2nd one. The first box is updated to be the same as the 2nd one, and since they're the same, there are no messages returned. It would be nice to label which one is start and which one is stop.