Can't get more XP after using /xp command.

When I use cheats to get XP, I can no longer get more XP by using cheats or collecting the XP dropped by entities.

Steps to Reproduce:

1. I type in "/xp 700" in chat using cheats
2. I get XP
3. I cannot get more XP

Observed Results:
I can no longer get more XP from anywhere.

Expected Results:
I expected I could get more XP!