Steps to reproduce the issue

1. Create a custom field from the type media and disable the permission to edit custom field value for the group Manager.
2. Edit in backend the article with this custom field as a super user and as an example select an image.
3. Login in frontend with the user in the group Manager and edit the article with this custom field and safe it without any changes.

Expected result

The article will be submitted and saved.

Actual result

This will show a warning with invalid field.

System information (as much as possible)

Tested on Joomla 3.9.10
with PHP 7.3.x, 7.2.x and 7.1.x
on Linux and Windows

Additional comments

Also the warning shown prints the field name to define the error field, which may be named differently from the label and therefore may not be findable.
I fixed this here #25626