Pandoc and Abbreviations

I'm trying to generate an ePub file with abbreviations. Here's an example: input.md:

---
title: Test
language: en
---

# Test

HTML is maintained by W3C.

and abbr.md:

*[HTML]: Hyper Text Markup Language
*[W3C]:  World Wide Web Consortium

and get-epub.sh:

#!/bin/bash
pandoc -f markdown+abbreviations -t epub\
--abbreviations="abbr.md"\
"input.md" -o "output.epub";

I'm expecting to see something like this in the output.epub file:

HTML is maintained by W3C.

while the actual output is:

HTML is maintained by W3C.

Did I miss something here?