How do I resolve an issue where a multislection widget has all preselected boxes?

When creating a multiselect grid tool it shows almost all boxes as pre-selected. This populates on the form when you attempt to view it and so no selection or deselection can happen.

The multiselect grid cells can be picked only once. That is its main feature. So when you load the form again, those selected will show the "selected" label defined in the widget settings.

If you want to clear the selected cells, you will have to delete the existing submissions in your form. Make sure to download the data for your local copy.