During issue creation, with the IssueService, and an IssueInputParameters bean, setting iip.setApplyDefaultValuesWhenParameterNotProvided(true) results in the Resolution Date being set to the same date as the Creation Date. Given that the workflow of the issue is Unresolved, I think that this should not happen:
The only scenario I can see that this flag would be usable is during an issue workflow transition to resolve. But its odd that no reference or check is made for such.
Without setApplyDefaultValuesWhenParameterNotProvided(true) issues do not inherit expected default values from Custom Field definitions. Addons would need to query every single custom field, to establish if it has a default value, then set it via IIP. If this is not a bug, what then is the purpose of that method?