When the worldborder expands slowly, it is possible to walk onto a block as soon as the border starts expanding into it. However, until the border catches up to your exact position within that block, you will take suffocation damage. This also applies to mobs, like bats flying into the border.
Expected behavior:
The area that players and mobs can move into is not rounded up to the nearest block.
Alternatively, the area in which players and mobs do not take suffocation damage is also rounded up to the nearest block.
Comments 7
Well, according to the description that you made, you said that the player can walk onto a block as the worldborder is expanding. Due to the player being partially outside of the block upon the worldborder doing this, they take damage.
I guess the fact that it lets you onto that block is the bug but it's a tricky situation because PART of the block is inside the border. The damage isn't a bug however.
This is intended.