Dear Catalina,
Thanks for your updates and kindness.
It seems that I can't reproduce the issue in our environment.
I create a content type Weekly Call Excel and a choice type of the site column Custom Column 1 and set the default value as metadata1 and add the column to the content type.
I then show the custom column Custom Column 1 on the library view.
Now when you add a new Excel file from the content type Weekly Call Excel, the column value should be metadata1 by default.
When you open the existing one>Info>Save a Copy>rename as a new file>Info>Show All Properties>change the metadata value and the updated value will be synced to the library. If you don't change, the default value will be synced.
Do you see that the metadata value isn't synced on the library view or you just see the blank value? If you see the blank value, do you set the default value on the site column?
Moreover, some possible reason may be that someone in the organization creates some flows updating the metadata value. Could you check whether there are some flows configured on the library to change the metadata value in the library automatically from your side?
Your effort and time are greatly appreciated!
Sincerely
Cliff | Microsoft Community Moderator