When using efficency to mine blocks, they dont break, they stutter, become "ghosts" but come back after, only comes into affect when insta mining, so efficency 3 or above with sand or efficency 5 and haste 2 for stone as two examples.

Link to video showcasing the bug.

https://1drv.ms/u/s!ArMCGapc8QeHh6ZZshKu_E2I4VFNcg?e=Sutmne