Intermittent WSOD when advagg used with filecache

Problem/Motivation

I am using filecache.
I notice that advagg is writing to cache.
I do not know enough about advagg under the hood because the files are ultimately written to public://files/js

Sometimes a WSOD occurs.
a page refresh brings it back but it is a bad user experience.
The directory, advagg/K, was created.

The website encountered an unexpected error. Please try again later.

Exception: Could not create cache folder /var/mycache/mysite/advagg/K in Drupal\filecache\Cache\FileSystemBackend->ensureCacheFolderExists() (line 349 of modules/contrib/filecache/src/Cache/FileSystemBackend.php).
Drupal\filecache\Cache\FileSystemBackend->set('KQdNROJyQlTOf-Uz6EmT3ZRZl6BO48TcTNiy3_ygm8E', Array, 1684342983, Array) (Line: 262)
Drupal\advagg\Asset\AssetOptimizer->scanFile(Array) (Line: 169)
Drupal\advagg\Asset\AssetOptimizer->processAssetArray(Array) (Line: 114)
advagg_css_alter(Array, Object, NULL) (Line: 562)
Drupal\Core\Extension\ModuleHandler->alter('css', Array, Object) (Line: 160)
Drupal\Core\Asset\AssetResolver->getCssAssets(Object, 1) (Line: 311)
Drupal\Core\Render\HtmlResponseAttachmentsProcessor->processAssetLibraries(Object, Array) (Line: 157)
Drupal\Core\Render\HtmlResponseAttachmentsProcessor->processAttachments(Object) (Line: 94)
Drupal\big_pipe\Render\BigPipeResponseAttachmentsProcessor->processAttachments(Object) (Line: 56)
Drupal\http2_server_push\Render\HtmlResponseAttachmentsProcessor->processAttachments(Object) (Line: 45)
Drupal\Core\EventSubscriber\HtmlResponseSubscriber->onRespond(Object, 'kernel.response', Object)
call_user_func(Array, Object, 'kernel.response', Object) (Line: 142)
Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch(Object, 'kernel.response') (Line: 202)
Symfony\Component\HttpKernel\HttpKernel->filterResponse(Object, Object, 1) (Line: 190)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 81)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 54)
Drupal\simple_oauth\HttpMiddleware\BasicAuthSwap->handle(Object, 1, 1) (Line: 58)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 106)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 85)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 43)
Drupal\webform_product\RedirectMiddleware->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 728)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes