if you break a respawn ancher after setting your spawn and then die you will come back in the overworld but every block you punch turns into an air or a nether block and if an air block is under you you will float and every block breaks instantly including unbreakable block.
Thank you for your report!
We're actually already tracking this issue in MC-175177, so I resolved and linked 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.
If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.
Quick Links:
📓 Issue Guidelines – 💬 Community Support – 📧 Customer Support – 📖 Game Wiki