If the Null is at the end, you need to do something to get the Beat Buddy to move to it, and then end. Here is what happens:
Intro - song, main loop - null, outro - null: Song starts with one press and plays through. When the song reaches its end, i.e. the end of the intro loop, it goes to the main loop where it will loop endlessly playing nothing. If you double tap to get to the outro loop, the song will end.
Intro - null, main loop - full song, outro null: Song starts with one press, plays the empty intro, and then progresses to the full song in the main loop. It will repeat the main loop over and over, so you have to be on top of it when the song ends and double tap to get to the outro, which is also empty, and will end the song.
Intro - null, main loop - null, outro - full song: One tap starts song, and advances to the main loop null, which loops over and over, playing nothing. When you double tap, the song “starts” playing the outro, The song plays fully through and then ends.
Personally, I like the first version, but you do need to double tap to end the song. Others like the third version, but the start which does not start the actual song part does not work well with loopers. Still others find option 2 acceptable, but the song then needs editing to add dead space to the end of the full song in the main loop to prevent unwanted sound when the song restarts.
So, yes, in many ways, it is just the software.