Thank you! That was enough of a hint to get me going. For others following along:
I changed my shortcode themes/mytheme/layout/shortcodes/ref_pg_abc.html from:
[A B C (ABC)]({{ ref "/pages/abc/home.md" }})
to
[A B C (ABC)]({{ ref . "/pages/abc/home.md" }})
Which solved the ref issue, but displayed text as opposed to rendering the markdown. So I changed the markdown file content/page/pg1/index.md from:
... The current {{<ref_pg_abc>}} user ...
to
... The current {{%ref_pg_abc%}} user ...