Design flaw, the join is not correct :(
hmm very sorry to see all the problems with these I'm going to have to research how to fix this :(
It's because they're still separate objects (See picture) - Some slicers can figure this out and combine them, but a lot don't and print them as two distinct objects pressed up against each other.
In blender these would be combined with a Boolean operation, not sure what 3d Builder would do, but you can probably search "how to join meshes" if you didn't figure this out already.
Edit: There are also some programs that can automatically fix these, like Meshmixer.