Probably due to the changes made for https://bugs.mojang.com/browse/MC-156013
Client velocity is reset (due to the new packet being sent) when you do the following:
move (so that the velocity reset becomes visable)
start digging a block (that isn't yet broken before switching to the next one in the next step)
while keeping the dig button pressed: look over to a neighboured block that is instantly breakable (grass, flowers, netherrack with an efficiency pickaxe, etc.)
-> velocity is reset
The easiest way to set this up is by going over a field with grass (or an effiency 5 pickaxe and netherrack, starting with a block that is not instantly mineable with a pickaxe -> not instantly mineable block next to an instantly mineable block)
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Adding my video from the duplicate bug report here too, to show how this can be easily reproduced by breaking crops and looking at the dirt below.
[media]
Is anyone actually still able to reproduce this in pre6? To me it looks like it's fixed.
Oh you're right, can't reproduce it anymore. Sorry then, just added the pre6 tag as the changelogs didn't make any mention of a change in this direction and it seemed too important to wait a day to test it.
So apparently this is fixed with pre6.