I have successfully reproduced this behavior in a Sandbox game. The core issue is the misleading visual feedback: when hovering the Blueprint, the game explicitly shows a ghost suggesting an underground belt connection. However, when I execute the Force Build, that suggestion is ignored and the ...

What did I do?
I tried to place a blueprint containing belts over an area that required an underground connection using "Super Force Build" (Ctrl + Shift + Click).
What happened?
When hovering the blueprint, the game correctly visualized a suggested underground belt connection (the ghost appeared ...