To recreate this bug, start mining a block in survival block and then open your chat box before the block breaks. Switch to adventure mode via commands. If you attempt to mine the same block, you can begin to mine it, but at the end of the breaking animation, instead of breaking the block, the animation starts over.
Linked issues
Comments

Duplicates MC-19676.
Confirmed for 1.14.4
It's kinda cool but def a bug
Here's a demo video
https://twitter.com/theqmagnet/status/1171564303524216832?s=20