I believe that the issue is in the select_multiple templates.
I changed the '==' to 'in' and that seemed to fix it.
<option value="{{ select.value }}" {% if select.value == field.value %}selected{% endif %}>{{ select.display_text }}</option>
<option value="{{ select.value }}" {% if select.value in field.value %}selected{% endif %}>{{ select.display_text }}</option>