I have found that:
fetch(url, { method: 'DELETE', credentials: 'include'}) will not send cookies for cross domain requests.
fetch(url, { method: 'POST', credentials: 'include' }) will send cookies correctly for cross domain requests.
Hmm, I'm guessing you are talking about client side requests, and this module simply proxies the github fetch polyfill in that case, and so if there's a bug in that behaviour it's probably in that library.
I have found that:
fetch(url, { method: 'DELETE', credentials: 'include'})
will not send cookies for cross domain requests.fetch(url, { method: 'POST', credentials: 'include' })
will send cookies correctly for cross domain requests.