I had mistakenly missed a function call.

src/components/FormWizard.vue

@@ -384,8 +384,8 @@
           return
         }
         let newTab = this.tabs[index]
-        if (newTab && newTab.afterTabChange !== undefined) {
-          newTab.afterTabChange()
+        if (newTab && newTab.afterChange !== undefined) {
+          newTab.afterChange()
         }
       },
       changeTab (oldIndex, newIndex, emitChangeEvent = true) {

test/unit/specs/FormWizard.spec.js

@@ -417,7 +417,7 @@ describe('FormWizard.vue', () => {
       const wizard = mount(threeStepWizard, {localVue})
       const wizardInstance = wizard.find(FormWizard)
       wizardInstance.vm.nextTab()
-      expect(threeStepWizard.methods.validationMethod).to.have.been.called()
+      expect(threeStepWizard.methods.validationMethod.should.have.been.called)
       expect(wizardInstance.vm.activeTabIndex).to.equal(1)
     })
     it('simple method on back navigation after change', () => {
@@ -427,7 +427,7 @@ describe('FormWizard.vue', () => {
       const wizardInstance = wizard.find(FormWizard)
       wizardInstance.vm.nextTab()
       wizardInstance.vm.prevTab()
-      expect(threeStepWizard.methods.validationMethod.called)
+      expect(threeStepWizard.methods.validationMethod.should.have.been.called)
     }) 
   })
   describe('with vue-router', ()=> {