Clicking on .aup file in Finder won't open it in Audacity

When I click on an .aup file in the Finder and Audacity is closed, Audacity opens automatically with the selected file, as I expect.

When I click on the same file in the Finder but Audacity is still active with another file or none, Audacity doesn’t react anyway. Opening the file with ‘Open’ from Audacity gives no problem. I expect it would open the file anyway.

Mac OS 10.15.7
Audacity 3.1.2

.aup files are set to be openend with Audacity

Thanks for the report.
This is a bug. The developers are aware of the issue.