I faced the same issue days ago, yet there was no duplicate content/link issue whatsoever.
If someone else faces this same issue, I'd recommend try the following method.
Managed to fix it within hours using this technique:
1. Open GSC => Settings => Crawl stats then click Open Report.
2. See the stats of your crawl budget at a glance, then scroll down to the By Purpose section.
3. Refresh means that the crawler returns again and again to check for new content etc otherwise the Discovery is what its name suggests. At this point, it's most likely that the Refresh percentage is higher than Discovery.
4. Click the Refresh report and see which page/URL has the highest percentage. It means that the page has the most frequent rate to be crawled. Now, all I had to do was put the URL that's discovered but not indexed as an internal link. (As an optional step, you can also temporarily put a URL that links to the URL that's discovered but not indexed as an internal link as well).
For e.g. the page which has the highest Refresh crawl rate is domain.test/highest-crawl-yoo whereas the discovered but not indexed page is domain.test/discovered-only-yoo. Then, just put domain.test/discovered-only-yoo as an internal link on domain.test/highest-crawl-yoo.
5. You can then manually fetch domain.test/highest-crawl-yoo on GSC again and request to index. It will tell Google that you have modified your content by adding an internal link, then crawl that internal link which is the discovered but not indexed.
In my case, I got that page index within one - two hours.