This only started happening recently. Possibly after a patch install. I was only contacted after the issue was found. I had nothing to do with server upgrades, so I am not entirely sure if they are the culprits.
There are several document libraries that use mandatory managed metadata fields. Users are asked to fill in these fields when they upload documents. This has been working for over a year without issue. Suddenly, in the modern experience, the managed metadata fields show as "Required info" error in the modern view. If you select one of the items and look at the details, the fields are actually all filled in correctly. If we switch to classic SharePoint, all the fields show up with the values as expected. I tried running the Product Configuration Wizard as I saw that there were some patches installed over the last couple of months. Found a missing feature and fixed that. But that did not fix this problem. I created a new Document Library and a new Term Set. Added a new field using the new term set and made it mandatory. Upload a document and properly set the field... same problem.
So, question is. Is there a bug? Is the site set up incorrectly? I have been using Managed Metadata for many years, this is the first time I see this issue.
This is a preview in modern view:
And this is the same group of files in classic view: