If a select element's "multiple" attribute is flipped at some point after the initial load, we'll collect formdata for that field assuming it's a multiple-select. But when we restore data for that field, it'll appear as a single-select (since the "multiple" attribute wouldn't have been changed yet), so we'll drop the data we collected.

We should investigate if it's feasible to manually flip the multiple attribute of the element when we notice that we have a MultipleSelect entry but !select->Multiple().