According to the multipart content-type spec, their is no requirement that there be a trailing line break on the close delimiter. I cross referenced this with the current apollo client implementation, which also does not look for a trailing line-break (anywhere in fact)

Overall, the body of a multipart entity may be specified as follows:
multipart-body := preamble 1*encapsulation
close-delimiter epilogue

encapsulation := delimiter CRLF body-part

delimiter := CRLF "--" boundary ; taken from Content-Type field.
; when content-type is multipart
; There must be no space
; between "--" and boundary.

close-delimiter := delimiter "--" ; Again, no space before "--"

preamble := *text ; to be ignored upon receipt.

epilogue := *text ; to be ignored upon receipt.

body-part = <"message" as defined in RFC 822, with all header fields optional, and with the specified delimiter not occurring anywhere in the message body, either on a line by itself or as a substring anywhere. Note that the semantics of a part differ from the semantics of a message, as described in the text.>

Currently, servers that do not add this additional line break (like [GraphQL-ruby pro]) result in the last multipart chunk not being recognized.