Extraneous space between characters/glyphs in groff eqn

I am running ubuntu 16.04 (it is on an OLD laptop). I am using 'pic | eqn -Tpdf -d@@ | groff -mm -Tpdf' software as per the ubuntu release. I need some symbols which are not in the standard groff TIMES font for the release. I have found the symbols I need in STIX and FreeMono that come with the ubuntu release. I have created the relevant font files for groff as per https://www.schaffter.ca/mom/momdoc/appendices.html.

It is almost working but there is extraneous spacing between characters / glyphs. See attached image. I have tried both STIX and FreeMono and they produce the same problem.

PDF output from groff eqn showing extraneous spacing:

The groff, eqn mark up for the imaged example above is:

.EQ
gsize +2
.EN
.EQ
xi ( tau ) = sigma sub { C ( tau ) } size -4 \[u2A1D] sub { f \[mo] Facts ( tau ) } f
.EN
.sp 1
.special STIXGeneralR
.EQ
xi ( tau ) = sigma sub { C ( tau ) } size -4 \[u2A1D] sub { f \[mo] Facts ( tau ) } f
.EN
.special
.sp 1
.EQ
xi ( tau ) = sigma sub { C ( tau ) } size -4 \[u2A1D] sub { f \[mo] Facts ( tau ) } f
.EN
.sp 1
.fam STIXGeneral
.EQ
xi ( tau ) = sigma sub { C ( tau ) } size -4 \[u2A1D] sub { f \[mo] Facts ( tau ) } f
.EN

How to I get rid of the extraneous spacing?

The first line does produce a [u2A1D] character not found message.

I think I have a workaround for you. I finally setup a test on my Fedora 25 with the STIX fonts and found that if you target for PostScript output instead of PDF, the eqn looks correctly spaced (to me), including the "join" bow-tie. You can then separately use ps2pdf (which comes with ghostscript) to convert to a good pdf. So try

pic <input | eqn -d@@ | groff -mm -Tps >text.ps
ps2pdf text.ps text.pdf