I believe the restock works, but the replacement sword is immediately destroyed, since the replacement sword disappears. I tested with 2 swords. Both disappear when the equipped sword breaks right after attacking an entity. The same happens with 2 axes.

In other words, the issue only occurs when a sword (or an axe, and probably any other tool) breaks right after you attack another entity. Perhaps due to some other methods that take place or due to a delayed state update when a tool is used to attack entities.

The sword gets restocked all right if if breaks when you are mining a block with a sword, for example.