Breaking blocks with any tool that has the unbreaking enchantment creates ghost blocks on the remote client causing suffocation and death whilst mining, digging and other activities.

Please see this video for details: https://youtu.be/SKU_MHwZop8

When playing on a vanilla single player world with a friend, any blocks broken using a tool with the unbreaking enchantment causes blocks to appear broken on the remote client, but locally the block still exists causing the remote player to take suffocation damage and die whilst inside blocks they cannot see.

The level of unbreaking determines the level of the issue.
I believe this is tied to the unbreaking enchantments "random chance" to take damage during use.
When the random chance chooses for the tool to not lose durability, the block breaking data is not sent to the local client and therefore the block does not break.

This works with pickaxes, shovels, axes, hoes enchanted with unbreaking.

Efficiency or Haste does not make a difference.