When breaking blocks with an empty hand if you pick up an item into your hand it resets block breaking progress. Also emptying your hand with q while breaking blocks resets the breaking progress.
Not sure about other affected versions.
MC-168942
Breaking blocks resets if in-hand item changes
Resolved
Duplicate
0
Th4V7
(Unassigned)
Unconfirmed
(Unassigned)
1.15.1
Retrieved
Also not sure if this is an intended feature to prevent people from breaking blocks then throwing their tool right before it breaks so the tool takes no damage but the block breaks just as quickly.