Sadly, you cannot trust what Google Search Console is showing you. It has the very annoying habit of adding new cases from its cached index into existing issues, while the validation is running (for up to 30 days). And newly added outdated index entries cause the running validation to fail immediately.
So it is common that one needs to trigger the validation multiple times, up to several dozens of times, or wait for some months, depending on the number of affected pages last crawled before the fix went live. You can verify that the entries which did cause the failure are fine when manually checking their live instance (and comparing the timestamps of the last crawl), and the fact that you can retrigger the validation without issues (otherwise the failed entry would fail the quick preliminary check that is done).
The issue is fully solved on Discourse side. It only takes some months of patience until each page in the Google index was truly updated.