After an arrow hits a block, it can keep some of it's velocity from when it was fired after the block is either moved or removed.
Steps to Reproduce:
Place a trapdoor at eye-level to the player
(Must be close enough to interact with the trapdoor)Crouch
Shoot an arrow at the trapdoor
As soon as the arrow hits it, open the trapdoor quickly after
Observed Results:
The arrow will hit the trapdoor, coming to a full stop. Once the player opens the trapdoor, the arrow will sometimes jolt forward with gained speed and height.
Expected Results:
After an arrow hits an obstruction and comes to a full stop, it should lose all of it's velocity, and fall straight to the ground if the obstruction is moved or removed.
Screenshots/Videos:
My demonstration of the issue, and reproducing it:
[media]
Notice how the first arrow comes to a full stop, and falls as expected, where as the second arrow gains lots of momentum seemingly for no reaon.
Related issues
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Can confirm. Also affects 20w51a, however, this seems intentional.
Can confirm in 21w03a.
Can confirm in 21w08b. Video attached.
Can confirm in 1.16.5.

Relates to [MC-180397]
Requesting ownership of this issue to maintain it

I think this is related to MC-265965
That issue is about in-game gravity pulling the arrow downward, this issue is arrows not properly removing their velocity after colliding with a block.

I found that it works with any block and that you don't have to sneak.
Cannot reproduce anymore with recent changes to arrows and projectiles.
Likely fixed due to the behavior mentioned in MC-278155.