Might be a bit difficult to explain this but here goes.

When you hold down the break block button while aiming at a series of breakable blocks, you can see that the hand swings more times than blocks are broken, whereas if they are destroyed in individual clicks, there's one hand swing for each block broken (sometimes less if done too fast but that's a separate issue).

Expected behaviour would be that when the block break button is held down, the hand would swing once each time a block is broken, instead of just swinging continuously throughout.

It seems as though between the breaking of each block there's a period of survival-like punching that takes place, which is also likely responsible for MC-9998.