I noticed this when I was in the nether. I was in a nether fortress farming blazes from the spawners, and the bar was working fine then. However, when I went to mine some quartz, the exp bar would not change whatsoever. It was stuck at around 60% towards lvl 29.

I restarted Minecraft and went back to the Overworld, but nothing happened. I get the exp just fine as my mending enchantment works, but my exp bar does not go up or down at all. I tried to do a level 30 enchantment but I couldn't so I'm fairly certain it's not a visual bug.

UPDATE:
It was the mending enchantment. It basically gave all experience to it. Never seen that before.