[Exim] Problem expanding added headers (v3.35)

I am trying to add a header that will be used as a flag in a later director.
The header is definitely added to the message (headers_add = X-Added-Header:
YES), but expanding it later via $h_X-Added-Header: or
$header_X-Added-Header: consistently returns <NULL>. Also
"def:h_X-Added-Header:" consistently expands to <FALSE> as well.

I've tried to dig up as much information on the headers_add option as I
could (I Read The Fine Manual, searched the Web, etc.), but there doesn't
seem to be much available.

Is this expected behavior, or am I doing something wrong?

Thanks. - John R. S.