The bug

For Mining Fatigue level 0 to 1, 1 to 2 and 3 to 4, the speed is reduced to 0.3x of the original speed. However, for level 2 to 3, the speed reduction behaves differently, slowing to 0.03x instead.

As an example, here's the table showing the breaking time of cobblestone by hand:

As seen in the table, the breaking times do not follow a geometric progression. For most levels, the breaking time increases by a factor of ~3.333 (1/0.3). At level 3, however, the increase is ~33.3 (1/0.03).

To reproduce

1. Place any block with a hardness greater than 0
2. Mine it with any tool which cannot instant break the block, and measure the time
3. Run the command

   /effect give @s mining_fatigue infinite 0

4. Repeat the above steps for the same block and tool, increasing the Mining Fatigue level in the command each time.

You can observe that the breaking time at level 3 deviates from the expected pattern.