The id is persisted when its generated the first time. Closes #2822. C…

…loses #2823

src/Forms/Submission.php

@@ -45,7 +45,7 @@ public function id($id = null)
     {
         return $this->fluentlyGetOrSet('id')
             ->getter(function ($id) {
-                return $id ?: microtime(true);
+                return $this->id = $id ?: microtime(true);
             })
             ->args(func_get_args());
     }

tests/Forms/SubmissionTest.php

@@ -0,0 +1,23 @@
+<?php
+
+namespace Tests\Forms;
+
+use Statamic\Facades\Form;
+use Tests\TestCase;
+
+class SubmissionTest extends TestCase
+{
+    /** @test */
+    public function the_id_is_generated_the_first_time_but_can_be_overridden()
+    {
+        $submission = Form::make('test')->makeSubmission();
+
+        $this->assertNotNull($id = $submission->id());
+        $this->assertEquals($id, $submission->id());
+        $this->assertEquals($id, $submission->id());
+
+        $submission->id('123');
+
+        $this->assertEquals('123', $submission->id());
+    }
+}