In my opinion, the bug is that these things can be copy and pasted to a new diagram without creating a duplicate.
We're not of the same opinion - but we've discussed this before.

If I'm generous, one fork or join being used in multiple places is confusing. I'm more included to describe it as disastrous.
Well, in my opinion, it's not
much more disastrous than any arbitrary shape being connected to other shapes by different lines in other diagrams. On any given diagram you're seeing a restricted view. For example, is it any
more correct to hide a line to another shape (and remove that shape) to or from the fork/join on the
only diagram it is on? I strongly would argue it's not. I agree these are not good practices, but...
Notes, text and boundaries are just properties of the diagram, so shouldn't really be re-used either. At least re-using them doesn't have modeling implications.
Obviously, they aren't - they are just objects like any other. The ONLY such object that
should be the property of the diagram is the title block, which conceptually is a component of the diagram and shouldn't be implemented in t_object anyway. Indeed, unlike the other types you mentioned, when you paste the object into the destination diagram, it takes on the values of the destination diagram rather than retain the values in the original diagram!
But it's Friday! So have a good weekend!
Paolo