I have updated the bug report to include the 26.52 hotfix in the affected versions.
I just checked v26.52 and the gaps are present. I also noticed that Obsidian in the inventory/hotbar shows the gaps as well, shown by the white pixel on the top right of the block.
It's not that the dragon has too much health. It just doesn't take the proper amount of damage from attacks, which is why the projectile is needed to correct the damage output. On version 1.0 the projectile is not needed due to the dragon already taking the correct amount of damage from attacks.
Still affects latest version.