Description:

If while mining you drop your tool(Pickaxe, Shovel, etc...) you will still continiue mining from the point you dropped your tool, which is normal. But if you finish it off with your hand, The block will disappear for a little bit and appear again for like 1-10 seconds(depending on how long will it take you to mine it with hand) and you can mine it again, but when you are about a quarter finished mining, the block will disappear and drop itself. Also, if you equip or pick up a pickaxe, the block will dissappear and drop itself.

How to reproduce:

In survival(Anywhere where you need time to break a block), find or place a block that is kinda fast to mine with iron pickaxe(Stone, Cobblestone, Ore).

Equip a stone pickaxe so it is slower a tiny bit than iron.

Mine until you see cracks(Better when halfway so it would take less time to mine using hand)

Drop your pickaxe using 'q'

Keep pressing left click so you mine the block with hand

When It breaks, The block will be as stated above^

Pickup the pickaxe or switch to another one

The block will disappear and drop itself

Repeat but with using a wooden shovel for slow speed and with a dirt block

The block will only reappear for about half a second because it is fast to mine with hand

If you have the patience(~150 seconds) try with diamond pick and obsidian it will reappear for long.

Screenshots(Sort by date):

1, 2, 3: Breaking block

4: Block Reappeared

5: After ~7 seconds the block disappeared and dropped itself

6, 7, 8: Mining the block again and it reappeared

9: Pickaxe picked up, block disappeared

Sorry I could not do the dirt, too fast for screenshots