I should have mentioned that a 90 degree rotation gets rid of the problem. However, it seems like there should be some diagnostic as this would appear to be a bad surface of some kind.
I am having trouble with this same problem and don’t know how to fix it. I have “flowed along surface” a set of 3D letters that cut through the outer surface of the target object. When I boolean union target object and text, one half of the target object becomes transparent. The same happens if I split and join. It would be great if someone could explain what’s happening and how to solve the problem. Collab Grip model.3dm (8.1 MB)
Your advice to “DivideAlongCreases > SplitAtTangents=Yes” certainly helped but the base problem was the TAPEDESIGN logo. After I cleaned it up, everything worked fine.