Steps to reproduce:
- Go to Site's Content & Data > Forms;
- Click to add a new form;
- Add a Grid field with 1 row and 1 column;
- Duplicate the field above;
- Click to publish and enter the form's URL;
- Mark the first grid's only option;
- Mark the second grid's only option.
The options on both grids should be marked.
Marking the option of one field unmarks the other field's option.