Delete that entry. Highlight it and press the minus button at the bottom of the listing. I’m guessing that if you do that it will switch back to the default Shift-Cmd-D.
I don’t even have Mail listed in that screen, so I am guessing that at some point you added it yourself - I think that’s the only way that those show up. (I only have Preview there, with “Rectangular Selection” set to Opt-Cmd-R, but I know that I added that.)
MsgFiler is a fantastic Mail plugin for quickly filing messages in mail folders which unfortunately will no longer work in Sonoma because Sonoma is not allowing Mail plugins anymore. It’s the main reason I haven’t tried upgrading to Sonoma yet.
I’ll try that if the annoyance gets to me to much. But Shift-CMD-D? Seriously? If I changed it I see why I did. That’s all on the left side of the keyboard. Seems like it would even more of a nuisance!
What I would suggest is either remove the custom key combo and see if the default key combo works reliably, or try using a different key combo. Either way, you will see if it has something to do with that specific key combo, or if it remains flaky with the default or another combo, then maybe something else is up.
Another idea is to create a new user account, set up your email address(es) on that user account, set up the custome key combo, and see if it works reliably there.
I don’t think that cmd-return is a system key combo, but who knows?