Hey folks!
I've burned so much time on this, I hope I'm just missing something silly
Intended Behaviour:
Once a given issue is transitioned to DONE, CLONE it. And update the Clone Issue's customfield (it's a "Time Stamp" field, in a Team-Managed Project), so that it's seven days from the triggerissue's same customfield's value.
Some points to clarify:
Rule Audit Log registers the automation as a "success".
I've tested and confirmed that the smart value works for copying the value. I've even tried adding a second step that does the "add seven days" action. The issue's History Log indicates that it copies correctly, but when the 'edit' action happens, it just erases the value.
I've tried appending a number of {{... .[dateformats]}} to the end, such as "jiraDate".
I've had no issues in the past doing this with a simple "Date" field type using smart values.
I've tried both the MyFieldName (seen below) and the customfield_12345 formats.

Any help here at all would be mana from the gods. Thanks Friends!