I am not sure if the below is a problem in harflatex. When I compile the below latex file
\begin{filecontents*}{\jobname.xmpdata}
\Title{test}
\Author{author}
\end{filecontents*}
\RequirePackage{harfload}
\documentclass[luatex,unicode,a4paper,12pt]{article}
\usepackage[a-2u]{pdfx}
\usepackage{fontspec}
\setmainfont{Segoe UI Emoji}[
RawFeature={mode=harf;+dist;+ccmp},
BoldFont={Segoe UI Bold},
ItalicFont={Segoe UI Italic},
BoldItalicFont={Segoe UI Bold Italic}]
\begin{document}
\section{Test}
Test 😃.
\end{document}
File | pdfa-test4.pdf
-- | --
Compliance | pdfa-2u
Result | Document does not conform to PDF/A.
Details | Validating file "pdfa-test4.pdf" for conformance level pdfa-2u
The Unicode for cid 9687 is unknown.
The Unicode for cid 9688 is unknown.
The Unicode for cid 9689 is unknown.
The Unicode for cid 9690 is unknown.
The Unicode for cid 9691 is unknown.
The document does not conform to the requested standard.
The document contains fonts without appropriate character to unicode mapping information (ToUnicode maps).The document does not conform to the PDF/A-2u standard.Done.
On the other hand, lualatex and the following latex file generated a PDF without validation error.
\begin{filecontents*}{\jobname.xmpdata}
\Title{test}
\Author{author}
\end{filecontents*}
\documentclass[luatex,unicode,a4paper,12pt]{article}
\usepackage[a-2u]{pdfx}
\usepackage{fontspec}
\setmainfont{Segoe UI Emoji}[
RawFeature={+dist;+ccmp},
BoldFont={Segoe UI Bold},
ItalicFont={Segoe UI Italic},
BoldItalicFont={Segoe UI Bold Italic}]
\begin{document}
\section{Test \textnormal{😃}}
Test 😃.
\end{document}