If “Defer Required Enforcement” property present at the form level is set to “yes” then the “When-validate-item” trigger set at the item level for a particular record would not get invoked.

The Defer Required Enforcement property at form level controls when required field validation is performed - it delays validation until commit time rather than at item level. However, it does NOT prevent the When-Validate-Item trigger from firing. The When-Validate-Item trigger still executes regardless of this form-level setting.