Enchant mending don't work correctly

I have a bug with my diamond pickaxes : I make 'efficiency II' and 'mending'. But my pickaxes don't repair.
I have make some tests With creative mode and The pickaxes I have enchanted with mending don't work correctly
-If I dig 1 experience block : no problem
-If I dig more of 3 block without experience after that I dig experience blocks : bugs appearing
-If I dig lot of block without experience after that I dig experience blocks : 'mending' don't work....

link of the video to see the bug: https://youtu.be/1B1XwzVM_GY

it still resides in 1.9.4

(sorry for my English I'm french)