If you put something you can use for reinforcement (like nether bricks) in your off hand and some in your inventory and use the offhand one to build in /ctf mode, 3 blocks will be consumed, 1 for reinforcement, 1 for building, and the other is just lost.
Before:

After:
Video:
https://streamable.com/z0ycee
If you put something you can use for reinforcement (like nether bricks) in your off hand and some in your inventory and use the offhand one to build in
/ctfmode, 3 blocks will be consumed, 1 for reinforcement, 1 for building, and the other is just lost.Before:

After:
Video:
https://streamable.com/z0ycee