You can delete and re-add the field to the form to fix the issue.
1.       Click Management
2.       Click DIY Forms
3.       Click Edit next to your page
4.       If the field is required *:
a.       Click the pencil icon next to the field
b.      Unmark the box for Required
c.       Click Update
d.      Click the red X to the right of the field to delete it
e.      Click Yes to confirm
5.       If the field is not required:
a.       Click the red X to the right of the field to delete it
b.      Click Yes to confirm
6.       Click Save
7.       Click Add Item +
8.       Click Add Fields
9.       Select the Category from the drop down menu
10.   Click the name of the field
11.   Click Update
12.   Click Save
13.   Click Go Live
14.   Click Yes, Go Live!
15.   Click Replace


If this does not resolve the issue, please contact support and provide the Article ID for reference to your issue.