Validators configuration is not posible for new 6UI complex fields such as info.magnolia.ui.field.MultiFieldDefinition and info.magnolia.ui.field.CompositeFieldDefinition. In 5UI they extend info.magnolia.ui.form.field.definition.ConfiguredFieldDefinition, which allows the usage of validators whereas in 6UI, they extend info.magnolia.ui.field.ConfiguredComplexPropertyDefinition, which does not contain the validators attribute.

This is a problem for customers migrating forms to 6 UI which contain validators for such fields.