We are able to "Add threat from existing..." in the Threats screen, but what is happening behind the scenes?
The objective of this article is to explain some rules logic which may help you understand and troubleshoot why certain behaviors may occur.
Whenever we add a threat from existing, it takes a copy of the threat, not a reference to the threat.
This means that it is no longer associated with the original library and the original threat, meaning any changes at a library level will not impact the copied threat.
This can be seen via the 'Manually created' tag on the threat.