Summary

After removing the "Done" resolution, we did observe that it was recreated. 
Reviewing the Audit Log, we could observe that it was created by a non-admin user with "Greenhopper Managed Resolution" Description.

Environment

9.6.0
.

Diagnosis

Check if after the resolution was removed, there was a Project created using the "Simplified Workflow".

Cause

The simplified workflow does expect to have a done resolution. When it does not find it, it does add it again.

This behavior is described on this ticket: JRASERVER-72691 - Getting issue details... STATUS . 

Steps to reproduce:
1. Create a new Project based on the Simplified Workflow, for instance, using SCRUM;
2. Add a new resolution "Done / Completed" on Jira Admin > Issues > Resolutions;
3. Delete the resolution "Done" moving all resolutions to "Done / Completed";
4. Edit the workflow (totally ignoring the warning to not modify the simplified workflow) and add at the top the Update resolution to "Done / Completed" for the transition to "Done";
5. Create an issue and transition it all the way until close it;
6. Issue was correctly saved with the resolution "Done / Completed";
7. Checking Jira Admin > Issues > Resolutions, no "Done" was recreated ;
8. Then, create a second project based on the Simplified Workflow;
9. After this project was created, the "Done" resolution was added again on Jira Admin > Issues > Resolutions . The workflow shows the new resolution added "The Resolution of the issue will be set to 10201." (The id for the removed "Done" was different).

Solution

The simplified workflow is based on a template, expecting some hard coded values (as the done)
If you want to use the simplified workflow, better use the default values, so instead of unifying all to "Done / Completed", use the default done.