Steps to replicate:

1. Create a project with a workflow that does not set the resolution at any point
2. Change the edit screen so that it has the 'resolution' field on it
3. Create an issue
4. Edit the issue to set a resolution

Result:

Issue does not have a resolved date

Expected result:

The issue has a resolved date and resolutiondate in the database has been updated.

Workaround

The resolution field should be altered either through a post-function, or by a screen specifically used to resolve issues. You should not have the resolved field on the edit issue screen.