diff --git a/lib/phpunit/bootstrap.php b/lib/phpunit/bootstrap.php
index dd15d1bb189..c7cf470686c 100644
--- a/lib/phpunit/bootstrap.php
+++ b/lib/phpunit/bootstrap.php
@@ -205,6 +205,20 @@ ini_set('log_errors', '1');
 $CFG->themerev = 1;
 $CFG->jsrev = 1;
 
+(function () {
+    // Determine if this test is being run with isolation.
+    // This is tricky because neither PHPUnit, nor PHP provide an official way to work this out.
+    // PHPUnit does set a value, but not until later on and we need this earlier.
+    // PHPUnit runs isolated tests by creating a class on the fly and running it through proc_open as standard input.
+    // There is no other legitimate reason to run PHPUnit this way that I'm aware of.
+    // When run in this way, PHP sets the value of $_SERVER['PHP_SELF'] to "Standard input code".
+    // It has done this since 2016, and it is unlikely to change.
+    define(
+        'PHPUNIT_ISOLATED_TEST',
+        $_SERVER['PHP_SELF'] === 'Standard input code',
+    );
+})();
+
 // load test case stub classes and other stuff
 require_once("$CFG->dirroot/lib/phpunit/lib.php");
 
diff --git a/lib/phpunit/classes/advanced_testcase.php b/lib/phpunit/classes/advanced_testcase.php
index 25ceef15107..eb7a9ae1a24 100644
--- a/lib/phpunit/classes/advanced_testcase.php
+++ b/lib/phpunit/classes/advanced_testcase.php
@@ -60,19 +60,6 @@ abstract class advanced_testcase extends base_testcase {
 
     }
 
-    /**
-     * Hook into the setInIsolation method to define an optional constant.
-     *
-     * @param bool $inisolation
-     */
-    public function setInIsolation(bool $inisolation): void {
-        parent::setInIsolation($inisolation);
-        if ($inisolation) {
-            // Note: This is safe to do because it will only be set once per test run.
-            define('PHPUNIT_ISOLATED_TEST', true);
-        }
-    }
-
     /**
      * Runs the bare test sequence.
      * @return void
diff --git a/lib/phpunit/classes/util.php b/lib/phpunit/classes/util.php
index fc131dc2614..0b0aec7eae6 100644
--- a/lib/phpunit/classes/util.php
+++ b/lib/phpunit/classes/util.php
@@ -320,7 +320,13 @@ class phpunit_util extends testing_util {
     public static function reset_database() {
         global $DB;
 
-        if (!is_null(self::$lastdbwrites) and self::$lastdbwrites == $DB->perf_get_writes()) {
+        if (defined('PHPUNIT_ISOLATED_TEST') && PHPUNIT_ISOLATED_TEST && self::$lastdbwrites === null) {
+            // This is an isolated test and the lastdbwrites has not yet been initialised.
+            // Isolated test runs are reset by the test runner before the run starts.
+            self::$lastdbwrites = $DB->perf_get_writes();
+        }
+
+        if (!is_null(self::$lastdbwrites) && self::$lastdbwrites == $DB->perf_get_writes()) {
             return false;
         }
 
diff --git a/lib/setuplib.php b/lib/setuplib.php
index 0a5af63d64e..47cc23030a6 100644
--- a/lib/setuplib.php
+++ b/lib/setuplib.php
@@ -2202,7 +2202,7 @@ function require_phpunit_isolation(): void {
         return;
     }
 
-    if (defined('PHPUNIT_ISOLATED_TEST')) {
+    if (defined('PHPUNIT_ISOLATED_TEST') && PHPUNIT_ISOLATED_TEST) {
         // Already isolated.
         return;
     }