I suggest you Export and Re-Import to create your copy, sorry its a pain, but can be due to different things like Flow attachments, Environment Variables and other weird scenarios.
It works for me on all apps - I vaguely recall seeing issues with large / complex apps or possibly browser issues somewhere in the past, however a workaround would be to Download a copy, then open another app (any app) and then choose Open (dots on right of main menu), open and save it with the new name.
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