We did some investigation on https://confluence.atlassian.com/jirakb/customers-are-unable-to-add-participants-to-a-service-desk-request-777027039.html for our plugin(ServiceRocket Salesforce.com Connector). We found out that any operation on the service desk screen(Share,Escalate, Resolve this issue, Cancel request) will evaluate validateFromParams function for all available custom fields even if the custom field is not added to the screen. We think this should not be the behaviour as the fields is not being added to the screen.
Sounds like a bug. Have you reported this to Atlassian Support? (Since we can’t create bugs anymore - that’s the only path to take).