NOSY_EMAIL_SENDING has the following interpretation:
...
email_sending – single
Controls the email sending from the nosy reactor. If multiple then a
separate email is sent to each recipient. If single then a single email
is sent with each recipient as a CC address.
...

I got confused with single addressee vs single email.
So the problem I'm having is related to NOSY_EMAIL_SENDING == multiple.

Corrected title.