Open attachment that contains forwarded email (MIME type message/rfc822)
Gmail cannot support this as I understand from FairEmail discussion on XDA (outlook mail client has related limitations).
There is a workaround, in case one is willing to have the forwarded email as text and not attachment.
OP had set mime_forward because they wanted to forward attachments per default
. If this is unset as per mutt documentation (to ask-no or ask-yes), the forwarded email is just text (not a message/rfc822) and the stock email app shows it fine as confirmed by them.