Why are 404 errors re-appearing, even after being marked as fixed?
A common issue we hear from users is regarding 404 and 410 errors that keep re-appearing in the Google Search Console, even after being marked as “Fixed”.
This happens because marking crawl errors as fixed in Google Search Console does not actually change anything in the indexing or crawling of your website. It’s purely a UI feature, which temporarily removes the error from your Google Search Console report.
Google will often re-crawl pages that return a 404/410 response. They do this to verify that the page is really gone and that the errors were not a temporary mistake on your site. This means that you can expect those URLs to keep re-appearing in your Google Search Console Crawl Error reports. You can find more information on this here.
You can also find more information on how Google deals with 404s/410s in general here.