.../rest/tests/src/Functional/ResourceTestBase.php    | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/core/modules/rest/tests/src/Functional/ResourceTestBase.php b/core/modules/rest/tests/src/Functional/ResourceTestBase.php
index d8f171e..4f435c0 100644
--- a/core/modules/rest/tests/src/Functional/ResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/ResourceTestBase.php
@@ -119,9 +119,7 @@ public function setUp() {
 
     // Ensure there's a clean slate: delete all REST resource config entities.
     $this->resourceConfigStorage->delete($this->resourceConfigStorage->loadMultiple());
-    // If we remove resources we also need to ensure they don't stick in the
-    // {router} table.
-    \Drupal::service('router.builder')->rebuild();
+    $this->refreshTestStateAfterRestResourceConfigChange();
   }
 
   /**
@@ -144,10 +142,23 @@ protected function provisionResource($resource_type, $formats = [], $authenticat
         'authentication' => $authentication,
       ]
     ])->save();
+    $this->refreshTestStateAfterRestResourceConfigChange();
+  }
+
+  /**
+   * Refreshes the state of the tester to be in sync with the testee.
+   *
+   * Should be called after every RestResourceConfig entity change.
+   */
+  protected function refreshTestStateAfterRestResourceConfigChange() {
     // Ensure that the cache tags invalidator has its internal values reset.
     // Otherwise the http_response cache tag invalidation won't work.
     $this->refreshVariables();
-    \Drupal::service('router.builder')->rebuild();
+
+    // Tests using this base class may trigger route rebuilds due to changes to
+    // RestResourceConfig entities. Ensure we generate routes using an
+    // up-to-date router.
+    \Drupal::service('router.builder')->rebuildIfNeeded();
   }
 
   /**