I'm sorry that you're having trouble with this feature. The use case you describe should work. It has just worked for me with a simple form, so it would be helpful to have a few details about the specific form that ended up in this situation.
When you select the "Reply to" drop-down, is the co-author not listed in the drop-down or is it listed but doesn't "stick" when you try to select it?
The order of events may also be relevant here: was the co-author's account already set up when you first shared the form, or did you first share the form to an email address and only then set up the account for that address? Did you share the form before or after selecting the "Send a receipt..." check box?
Also, do you have a field selected in the "Send to" drop-down? Did you, at any point in time, select the "Don't include..." check box below the "Reply to" drop-down?
Co-author is not listed in drop-down at all, under my login which is the administrator AND if I login as the co-author to edit the form. In both cases- co-author email address does not appear in drop-down to select.
Co-author account was setup 3 months prior to sharing this form yesterday. when I shared the form. Shared the form after selecting "Send a receipt", but also removed that check mark as administrator and logged in as co-author and there was still no option to use co authors email address.
The don't include check box was never checked.
A note about the form: it's a simple dog adoption form for a dog rescue. Why can't we just add an email address manually in this field and why does it take all these "co-authors and sharing" to accomplish a different reply to email address in that field. Adobe has out-thunk itself on this one.
The address selected in the drop-down may be used as a Reply To address for many automatically-generated email messages, so whoever has that address should have control over this setting... which is what sharing the form to them as co-author accomplishes.