Not gaining any xp

When I kill a zombie or any other mobs it' doesn't drop any xp points.

*Expected result.
Kiling zombie or any other mobs should give you xp points.

*Actual results
Not gaining any xp from killing mobs.

This bug only have affected one of my survival world. And rest of my world are working fine .

I hope you will solve my problem as soon as possible.
Thank you.