I crafted about a stack of concrete powder in my survival world and wanted to turn it into concrete. I went to some water and made a stack out of the blocks. I went to the bottom and started mining the concrete. After I had finished, there was some concrete powder left flying in the sky.

Edit: Most of the powder broke and fell down. Since this was quite a while ago, I don't really remember that there was some flying concrete powder. I will try testing this out again in 1.9.

Re-confirmed in 1.16.40 Hotfix on Windows 10. Confirmation was in a single-player world (Multiplayer toggle off).

Steps to reproduce:

1. In 1-deep water, place a block of concrete powder. Stack about 64 more concrete powder on top of it.
2. In survival mode, mine the bottom concrete block with a pickaxe, and keep using the pickaxe as new blocks of powder fall into its place.

Expected results:
As you mine blocks of concrete, the concrete powder blocks above them fall straight down as gravity blocks, replacing the mined block and turning into concrete. After collecting the floating mined items, you have about 65 blocks of concrete.

Actual results:
Some of the powder above the block you're mining is ejected sideways from the stack into the water and is not converted to concrete. After collecting the floating items, you have about 32 blocks of concrete and about 32 blocks of concrete powder. There may also be ghosts of some of the powder blocks in their pre-mining positions above you; they disappear if you try to interact with them.