Blocks reappear after being broken too fast.
This occurs most often with shovels and mining sand or gravel.
To replicate use efficiency 5 and haste 2 and just hold down the break button, whilst moving side to side. This has happened on many worlds but I don't know if it's lag.
Thank you for your report!
We're actually already tracking this issue at MCPE-48914, so I will resolve and link this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
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