The Bug:
Breaking blocks that can be instantly destroyed slowly breaks the targeted block behind them.
Please note that this is not a visual glitch. The targeted block will be destroyed as normal once the breaking animation on it is complete.
Steps to Reproduce:
Obtain a stack of short grass and switch to survival mode.
While looking at some dirt, constantly place and break the short grass in quick succession.
Observe the block breaking progress of the targeted block behind the short grass.
Observed Behavior:
The targeted block behind the block that can be instantly destroyed slowly breaks and shows block cracks.
Expected Behavior:
The targeted block behind the block that can be instantly destroyed would not break at all.
Linked issues
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The specific snapshot that apparently introduced this behaviour was Minecraft 1.14.4 Pre-Release 6.
Reopened since this behaviour didn't exist prior to 1.14.4-pre6, as opposed to MC-69865.

Some (maybe distracting) information to add:
The rate at which the block breaks increases if the player is wielding the appropriate tool for the block. It's not always 1 "point" of crackage.
This only works with blocks that are naturally instantly breakable with an open hand; I have not been able to recreate this using efficiency tools on dirt, shears on leaves, or by trying to hit a block through chickens.

This issue becomes very obvious when holding right-click to use bonemeal to grow crops while holding the replacement crop in off-hand. (ie: <hold right>bonemeal, <still holding right>bonemeal, <still holding right>bonemeal, <click left>break , <still holding right>place new crop, <still holding right>bonemeal... and so on). The farmland becomes damaged over time, and eventually breaks.
The "damage" to the farmland does not go away unless the farmland is hit exactly once, directly, without holding right-click, at which point the damage resets.
i could reproduce this only because my mouse was broken lol
after a few days i changed my mouse
it can be reproduced in 1.12.2 & 1.15.2
Reproduced in 1.16.1
This has been partially fixed in 1.16.3. The block still goes through the breaking animation, and freezes the animation if you stop prematurely, however the actual block breaking upon completion of the break block animation (the cracks through the block), does not result in a broken block. I cannot find the bugfix that directly solved this issue.
Though I'm weary to call this issue completely fixed. There is still the problem of the block breaking animation occuring. And the fact that it remains until the block is interacted with again.
Can confirm in 21w03a.
Can confirm in 21w11a.
Can confirm 1.17
Can confirm 1.17.1 Pre-Release 1; I was wondering why my farmland kept breaking during bonemeal frenzies.
Note: I reported partially broken blocks staying partially broken back in 1.8 (since 1.7.x didn't have the behaviour). It was ignored then and it's been ignored in the 7 years that have passed since then. Don't expect a quick resolution to this.
1.17 in a realm this issue happens. Made a fast farm. (Dispensers with bone meal and a redstone clock.) farmland will start displaying animation as I fast farm. farmland usually breaks but not always. If you look away from the farmland for a few seconds the breaking animation disappears.
Can confirm in 1.17.1.
I have this issue in 1.17: a sweeping edge killing chamber, where while i'm afking the dispenser that puts the minecart in for me to hit it keeps breaking time after time

Can confirm in 21w44a.
Can confirm in 1.18.1.
Can confirm in 1.18.2.
Can confirm in 1.19

can confirm in 1.20 Pre-release 1

Can confirm in 1.19.4
Can confirm in 23w44a

I found out, in which version exactly this bug was introduced. In version 1.14-pre5, this bug doesn't occur, in version 1.14-pre6, this bug occurs
can confirm in 1.21
Thank you for your report!
We're actually already tracking this issue in MC-69865, so I resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.