No where does it say that filling the bar guarantees an upgrade (this, or refine, or BA)
It is said that as the bar fills up, your chance of upgrade increases, and therefore strongly implied that once the bar fills, the next upgrade should be 100%.
It is a rare occurrence of horrible bad luck thou, if the small number of complaint is anything to go by (and I have yet to personally encounter it) Thou it's possible that some people don't post about it as they realized that this isn't a bug.