From Barry Wand (email@example.com):
The user is in an ACL as a member of a Group that has Editor access to a database and has been included in a Role that is authorized to modify a field in a Controlled Access Section. The user is also listed explicitly with Designer access. The user still cannot edit that field although other people in the same group can.
Explicit ACL entries always override Group document entries. If your Group entry has the Role assigned but your explicit entry does not then you do not have the Role assigned to you. In this case you have Designer access but without the Role assigned. You will need to adjust the ACL to assign that Role to your explicit entry also.