A custom field may be sensitive to the project context of the issue currently being validated. For instance I would like to get the list a actors associated with a project role of the project referenced by the issue being validated. The CustomFieldType interface method "public void validateFromParams(CustomFieldParams relevantParams, ErrorCollection errorCollectionToAddTo, FieldConfig config)" does not pass the Issue reference so I can't determine the projectId. I have tried different hacks to get access to the current issue (i.e. getting the "EditIssue" action from the Action context. The problem here is that if you edit another issue in another browser tab of the same browser, the second issue will overwrite the "Edit Issue" Action before the first Issue is validated.
If system field types get an Issue reference in the validation method why don't custom field types.