False alarm! Not an emoji parsing issue. 🏠 is coming through fine, it's just attached to https://tantek.com/? with a trailing question mark. I was confused because php-mf2 evidently strips the empty ? and merges it into the other https://tantek.com (without ?) rel-url.

Is that merging the correct behavior? Should mf2py do it? @tantek @kevinmarks 🤷🤷🤷

The following HTML:

<a href=/? rel=foo>bar</a>

with URL: https://example.com

parses on php.microformats.io to:

{
    "items": [],
    "rels": {
        "foo": [
            "https://example.com/"
        ]
    },
    "rel-urls": {
        "https://example.com/": {
            "text": "bar",
            "rels": [
                "foo"
            ]
        }
    },
    "debug": {...}
}

and on python.microformats.io to:

{
 "items": [],
 "rels": {
  "foo": [
   "https://example.com/"
  ]
 },
 "rel-urls": {
  "https://example.com/": {
   "text": "bar",
   "rels": [
    "foo"
   ]
  }
 },
 "debug": {...}
}

Considering the original problem is no longer applicable I'm going to close this issue.