Hi David, it was upon trying to reproduce the problem that I both succeeded on creating the issue and solving it! Turns out if I create my email hyperlink through the Hyperlinks panel (the option where you can specify a subject)it works fine, but will not duplicate properly. If you set the email URL with Go to URL in Buttons and Forms, then the button duplicates perfectly! Although I am not sure you can hard code in a subject that way.

To illustrate this point I recorded my creating a button both ways, one working and one not (apologies, my video is terrible compared to you who is so good with video tutorials… apologies in advance)

If you can get through my mundane 6 min video you will see the issue reproduced and then redone correctly. I am fairly certain this was not an issue pre-CC2020 but I could be wrong. Surely I have not been creating buttons incorrectly for 20 years!