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

It pains me to resolve this since I strongly dislike this behavior, but sadly it is intended (MCPE-30610)
This isn't a bug, Minecraft Windows 10 places blocks in a line.