[web2py] bug? - "list:string" widget changes from multiple select to separate option list

[Carlos]

Hi,

I've noticed that a "list:string" field, after a failed submit (with
validation errors) and when at least one value is selected, changes
from multiple select widget to the separate options list widget.

   Field('test', 'list:string', requires=IS_IN_SET(['a','b','c'],
multiple=True))

This occurs with new (create) and existing (update) records.

Is this a bug?.

Thanks,

   Carlos