Jsut went thourgh this exat scenario an a smaller scale helping @Iquo Inyang here. So here's my assessment.
She had a smaller list of 404 that were submitted for validation. GSC returned a result that the first 5 tested had failed. And it left the rest of the list in "pending" status for over three weeks.
So my assumption is when you submit a 404 for validation, GSC expects that something other than a 404 should be returned when retested. Otherwise, why would you be re-validating?
When it reaches 5 tests and they all fail, it puts the rest of the list into pending mode and stops trying. (Interesting that in both our cases, it quit after exactly 5 errors, despite huge difference in the number of URLs submitted.)
In our case, the redirects that were to solve the 404s had gotten lost in the shuffle and hadn't yet been implemented, so the validation failures were totally legit.
The needed redirects have been implemented and she's going to resubmit for validation. Expect those will go through fine. We're also submitting a temporary XML sitemap of the 404'd URLs to help G discover them more quickly.
So I'd strongly suggest, if you haven't already, that the list of 1326 404d URLs be run through Screaming Frog in List mode to confirm that they are all in fact redirecting properly first.
So my experience (with a sample size of 1) indicates - the failed status says "GSC expected a change in status from 404 to something else but the new crawl still returned the 404 so whatever fix you think you implemented isn't working".