This sort might be very useful when mixing opaque objects with transparent objects for example. This project is open source, you can look at the source code of the classes extending AbstractRenderBucket.
No this is not a bug, this is the expected behaviour. If I had been able to reproduce it, I would have tried to fix it. I'm not a magician.
There's a user guide and some online API documentation, you already know that but I admit that it's a bit slim. I'm alone, it's an open source project, I'm only a volunteer, I do what I can on my spare time.
If it was really a non deterministic behaviour, I would be really worried.