After updating the diff module to 8.x-1.0-rc1, I get the following in my log:
Notice: Array to string conversion in Drupal\diff\Plugin\diff\Field\ImageFieldBuilder->build() (line 70 of /home/[anonymized]/public_html/modules/diff/src/Plugin/diff/Field/ImageFieldBuilder.php)
#0 /home/[anonymized]/public_html/core/includes/bootstrap.inc(548): _drupal_error_handler_real(8, 'Array to string...', '/home/[anonymized]...', 70, Array)
#1 [internal function]: _drupal_error_handler(8, 'Array to string...', '/home/[anonymized]...', 70, Array)
#2 /home/[anonymized]/public_html/modules/diff/src/Plugin/diff/Field/ImageFieldBuilder.php(70): implode('\n', Array)
#3 /home/[anonymized]/public_html/modules/diff/src/DiffEntityParser.php(94): Drupal\diff\Plugin\diff\Field\ImageFieldBuilder->build(Object(Drupal\file\Plugin\Field\FieldType\FileFieldItemList))
#4 /home/[anonymized]/public_html/modules/diff/src/DiffEntityComparison.php(96): Drupal\diff\DiffEntityParser->parseEntity(Object(Drupal\node\Entity\Node))
#5 /home/[anonymized]/public_html/modules/diff/src/Plugin/diff/Layout/SplitFieldsDiffLayout.php(115): Drupal\diff\DiffEntityComparison->compareRevisions(Object(Drupal\node\Entity\Node), Object(Drupal\node\Entity\Node))
#6 /home/[anonymized]/public_html/modules/diff/src/Controller/PluginRevisionController.php(167): Drupal\diff\Plugin\diff\Layout\SplitFieldsDiffLayout->build(Object(Drupal\node\Entity\Node), Object(Drupal\node\Entity\Node), Object(Drupal\node\Entity\Node))
#7 /home/[anonymized]/public_html/modules/diff/src/Controller/NodeRevisionController.php(49): Drupal\diff\Controller\PluginRevisionController->compareEntityRevisions(Object(Drupal\Core\Routing\CurrentRouteMatch), Object(Drupal\node\Entity\Node), Object(Drupal\node\Entity\Node), 'split_fields')
#8 [internal function]: Drupal\diff\Controller\NodeRevisionController->compareNodeRevisions(Object(Drupal\node\Entity\Node), Object(Drupal\node\Entity\Node), Object(Drupal\node\Entity\Node), 'split_fields')
#9 /home/[anonymized]/public_html/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(123): call_user_func_array(Array, Array)
#10 /home/[anonymized]/public_html/core/lib/Drupal/Core/Render/Renderer.php(574): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
#11 /home/[anonymized]/public_html/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(124): Drupal\Core\Render\Renderer->executeInRenderContext(Object(Drupal\Core\Render\RenderContext), Object(Closure))
#12 /home/[anonymized]/public_html/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(97): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array)
#13 [internal function]: Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
#14 /home/[anonymized]/public_html/vendor/symfony/http-kernel/HttpKernel.php(139): call_user_func_array(Object(Closure), Array)
#15 /home/[anonymized]/public_html/vendor/symfony/http-kernel/HttpKernel.php(62): Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object(Symfony\Component\HttpFoundation\Request), 1)
#16 /home/[anonymized]/public_html/core/lib/Drupal/Core/StackMiddleware/Session.php(57): Symfony\Component\HttpKernel\HttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#17 /home/[anonymized]/public_html/core/lib/Drupal/Core/StackMiddleware/KernelPreHandle.php(47): Drupal\Core\StackMiddleware\Session->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#18 /home/[anonymized]/public_html/core/modules/page_cache/src/StackMiddleware/PageCache.php(99): Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#19 /home/[anonymized]/public_html/core/modules/page_cache/src/StackMiddleware/PageCache.php(78): Drupal\page_cache\StackMiddleware\PageCache->pass(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#20 /home/[anonymized]/public_html/core/modules/ban/src/BanMiddleware.php(50): Drupal\page_cache\StackMiddleware\PageCache->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#21 /home/[anonymized]/public_html/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php(47): Drupal\ban\BanMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#22 /home/[anonymized]/public_html/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php(50): Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#23 /home/[anonymized]/public_html/vendor/stack/builder/src/Stack/StackedHttpKernel.php(23): Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#24 /home/[anonymized]/public_html/core/lib/Drupal/Core/DrupalKernel.php(652): Stack\StackedHttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#25 /home/[anonymized]/public_html/index.php(19): Drupal\Core\DrupalKernel->handle(Object(Symfony\Component\HttpFoundation\Request)) #26 {main}.
Needs review
1.0
Diff builder plugins
The change is currently missing an automated test that fails when run with the original code, and succeeds when the bug has been fixed.
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
I am seeing this locally as well and the patch in #17 still applies to the latest code in 8.x-1.x branch and the PHP notices go away.