I found a similar wierd bug in the makebottle tutorial. I think your case has something with my case....
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Recently, my colleague tried the makebottle tutorial.
She followed the instruction in the tutorial and found the final part of it to be mysterious, for in the above codes it instructed to use Boolean operation to fuse parts of the bottle, but in the final part it finished with the BrepBuilder to make a compound of the thread and the bottle body.
She tried to use boolean fuse to make one solid bottle with those parts, but failed doing so.
The result of fusing of the thread and the body is just the thread, not a whole bottle with the thread.
I think the developer who made the tutorial already knew about this bug, so he avoided using boolean operation in the finalizing the bottle, and just finished it with making a compound.
Is my guess is right? and does the bug still exist in the latest version of OCC?