If you have created custom user fields of type Text (255 char max), "Visible to user, teachers and admins" (visible=3 in table user_info_field) and you're in a course, and export participants list :

• if you are Administrator, user custom fields are well included in the Excel export
• but if you are Teacher, user custom fields are NOT included in the Excel export

This can be corrected by making a small change in /user/profile/lib.php file, in the function is_visible, under the "case PROFILE_VISIBLE_TEACHERS" part, by adding one more condition just before the "else" part :

                }
                  else if (has_capability('moodle/site:viewuseridentity', $context)) {
                    return true;

Hope this can be corrected soon.

I don't do Moodle programmation and don't have Git repository, but it should be very simple to do.

For french speaking people (or see how code is included), see https://moodle.org/mod/forum/discuss.php?d=461746#p1856027