The block break animation renders at the incorrect rate under (at least) two circumstances:
1: You start breaking the block while in air
2: You change tool during use

To repeat:
1:

• Jump
• while in air try breaking a block.
• The break animation's speed is incorrect
  -- View attachment: 'Video1.wmv'

2:

• Start breaking a block with your hands
• Change item (while still breaking the block)
• The block break animation will be incorrect for the new item (resumes with old tool's rate)
  -- View attachment: 'Video2.wmv'