When mining a block in survival mode, if you go into spectator mode(F3+n) then the block you are mining still has the breaking animation on it, although the animation is on a loop and does seem to be a little slow than normal

Ways to replicate :
1. Start breaking a block
2. Before you break the block, press F3+n to go into spectator mode, dont stop breaking the block

If you're still holding the "break" button aiming at the block you were breaking, the mining animation plays on top of the block but much slower and its on a loop