Dear Martin,
Welcome to the forum here.
I do a test and it seems that I can't reproduce such an issue.
In my environment, I create a document content type naming Document 2 Type A and add the columns User Field 3 and User Field 4 and set the content type as the default content type.
Then when I go to the Form Settings, I can see that the option can be activated. I kindly suggest you click on Customize in Power Apps>click on Edit fields in Fields> add the content type fields to the original customized form and then publish the form to the site library to see the result.
Meanwhile, check whether the issue only happens in the specific site library in your environment. If yes and if it is convenient, I suggest you copy or move the data to the new library and use the new library as a quicker fix.
Best Regards,
Cliff