When the machine name of the entity type bundle is a pure number, the extra fields created will not be displayed in the manage display and form manage display, or in other words, they cannot be found.
repro steps
1.Create a content type use a pure numeric machine name. (For example, 2025) (/admin/structure/types/add)
2.Create any extra fields in this content type.(Name is arbitrary)
3.Go to see the manage display and form manage display page, You won't see any extra fields.
4.The same steps are performed in the article type, It will be displayed as expected in the corresponding manage display.
Active
2.1
Miscellaneous