There is DEFINITELY some funny business going on with donut bonus selections.

I'm fairly sure there's a pattern, but I'm even more certain that it's not worth the bother of figuring out for sure whether there is. (I'm reminded of Springfield Downs here.) Even if you could identify a pattern, you'd then have the problem of figuring out where you are in the series, leaving you in more or less the same place as if the sequence were entirely random.

Just play and enjoy your well-earned donuts.