Steps to reproduce the issue

1. Create user custom field, can be a simple text file
2. Login to backend of your site using a super admin account, edit your account, try to enter value for the custom field you created

Expected result

Custom field data being saved properly

Actual result

The field data look like saved properly but it is not. If you try to view your profile from frontend, you won't see the data you entered

System information (as much as possible)

Joomla 3.7, updated from stagging today

Additional comments

While trying to look at the code, I see that while saving user data, after this line https://github.com/joomla/joomla-cms/blob/staging/plugins/system/fields/fields.php#L111 , $fieldData is not an array (as the plugin code expect), it is actually a json string (maybe it is loaded directly from params fields in #__users table)

@Bakual , @laoneo Please check as you two probably understand custom fields system better than most of us here