Ajax :update used when redirect_to action

you really don't want to mix your rendering like that. and there is no
need to use :update => 'mypropForm' in the ajax call in your view, as
you are doing a page.replace_html.

you're making an ajax request, so instead of redirec_to, you should do

render :update do |page|
  if @proposal.save
    page.redirect_to ...
  else
    page.replace_html 'mypropForm', :partial => 'myprop_form'
  end
end