we have the same issue and we opened a case with Microsoft about this. unfortunately they reported this is by design because they have changed the secondary BDC string fields to multi-line text. unfortunately they don't seem to have realized that this breaks important functionality.
see: http://social.msdn.microsoft.com/Forums/en-US/sharepointbdc/thread/911c155a-32bb-4504-9292-6a604a0e1626
also, if you can it would help to open another case with Microsoft which maybe will help to convince them this needs to be resolved!
thanks,
jeroen