example form definition Json
{"maxLength":100,"placeHolder":"ClientName","id":"field_90519299646713E11","name":"ClientName","label":"ClientName","required":false,"readOnly":true,"validateOnChange":false,"helpMessage":"","binding":"ClientName","standaloneClassName":"java.lang.String","code":"TextBox","serializedFieldClassName":"org.kie.workbench.common.forms.fields.shared.fieldTypes.basic.textBox.definition.TextBoxFieldDefinition"}
which has readOnly = true.
In rg.kie.server.services.jbpm.ui.form.render.model.FormField,
@JsonProperty("readonly")
private boolean readOnly;
which check readonly instead of readOnly.
Easy fix would be change
@JsonProperty("readonly")
to
@JsonProperty("readOnly")