I've noticed this in 1.1 but never reported it. I noticed it in 1.2 Beta 1; it was more apparent because I got the chime when I hit XP level 15.

My XP bar showed I was level 14 and most of the way to level 15. I got some XP from killing mobs. I got the chime saying I hit an XP level which is a multiple of 5. My XP bar still showed 14.

When I hit Escape and then Resume it showed the correct level.

I was recording at the time, you can see here: https://youtu.be/5csGJNtgUa4?t=18m13s

You can see the same thing at 14:36 in the video when I am at 13 and picking up XP. It does update to 14 at a weird time at 15:15 when I think I am not picking up any XP.