Don't know if this will help, but went to the link, and got same results 
with converting to pdf resulting in no emoji..
With the text example thou, I was able to get something that worked..
Saved the file as an html file.
Then opened the file in firefox.
Then printed to file on my Fedora 32 machine to a PDF.
That PDF file does seem to have the kiss_mark emojis.
Don't know if it will work  with the ones you want, but might want to try 
that...
On 24 Jan 2021 at 21:16, Jeffrey Walton wrote:
Send reply to:          noloader@gmail.com
From:                   Jeffrey Walton <noloader@gmail.com>
Date sent:              Sun, 24 Jan 2021 21:16:16 -0500
Subject:                [libreoffice-users] lowriter does not handle emoji's 
properly
To:                     users@global.libreoffice.org
Hi Everyone,
I need help to workaround
https://bugs.documentfoundation.org/show_bug.cgi?id=139892.
I have an ODT document with an emoji in it. An emoji is just a Unicode
codepoint (https://unicode.org/emoji/charts/full-emoji-list.html)
After conversion to PDF using lowriter the emoji is not present.
Conversion happens with:
    lowriter --convert-to pdf emoji.odt
I'm working on Ubuntu 20.04 (Focal), x86_64, fully patched. Ubuntu
20.04 provides LibreOffice 6.4.6.2 40 (Build:2).
How do I ensure the emoji survives the conversion?
Thanks in advance,
Jeff
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