When I break a block (wood, stone etc), I expect the black pixels to slowly destroy the block.
But instead, black lines appear and just move over the object as if they were an overlay and then the block breaks. So far, this has only happened when I break a block from beneath, for example when I`m standing under a tree.

To recreate the bug, create a Singleplayer Survival World, craft a wooden pickaxe or axe (the slower the block breaks, the better the bug is visible) and stand under the blocks you try to mine.