Steps to reproduce:

1. Set gamemode to creative to make this easier to reproduce;
2. Enter fly mode;
3. Place a block on the ground and then place another one right next to it while still holding the place block button;
4. Go to another area (still holding the button), you won't place anything there;
5. Go back to the same line (still holding the button) you started and you'll be placing blocks again (locked in that exact line).