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 2
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.