Hello!
The building I am trying to print includes many slabs and when the printer is placing them, first it puts them correctly then after some time puts another slab on top of all the other ones, making them full blocks. I tried on both singleplayer, and multiplayer, the issue still occurs.

I demonstrated the issue in this video:

I am using;

• Forge 1.12.2 - 14.23.4.2745
• OptiFine 1.12.2 HD U E2
• LunatriusCore 1.12.2 - 1.2.0.42
• Schematica 1.12.2 - 1.8.0.169

Thank you!

This is an issue that can't be fixed if that is a 1 block wide wall. It tries to place the slabs correctly, but because of the placement logic that means it will right click on the slab bellow, making it fail.

Actually a duplicate of #358.