It's really simple, it goes like this, if your using a carrot on a stick to control a pig that your ridding and you right click to let the pig eat it and get a boost, it will then lose durability like always... But if you press F to swap hands, the carrot on a stick will gain back the 7 points of durability it lost by using the boost.

These will all bring back the durability:

• Pressing F to swap hands
• Picking it up in the inventory GUI
• Dropping it, then picking it up

First started happening in 1.9 snapshot 16w35e, which was just after MC-87567 (crash when using carrot on stick) was fixed.