It's not a bug. Most people don't need to fill it up but some people have to. It's just bad luck since some people have said on this forum that filling the bar for either doing refinement, battle armor, and charms doesn't mean you get when you fill it up. I, myself, filled my bar for refinement and thought it was strange but saw posts about it. 31/30 guarantees a level up rather than 30/30.