vmod_header can be used in vcl_backend_fetch?

[matrix]

I would like to control bereq header below.(varnish4.1.8)

header.append(bereq.http.Accept-Encoding, "aaa");
header.append(bereq.http.Accept-Encoding, "bbb");

But bereq.http.Accept-Encoding header is "aaa". (not "aaa,bbb")
is it possible to handle the bereq using vmod_header ?

or is it varnish-modules bug?

Best regards.