I couldn't reproduce the behaviour locally but debugging the PDF export code shows that for an image attachment with this reference
[[image:My Space.My Page@my image.png]]
the URL generated by the PDF URL factory is
which is decoded by PDFURIResolver, our custom URI resolver that is used with FOP, into
/xwiki/bin/download/My Space/My Page/my image.png
and is not found in the map of (URL, AttachmentReference) build by the PDF URL factory, because the key is always an encoded URL.
It seems this is a regression introduced by
XWIKI-9437. Starting with 5.2RC1 the URLs created through the URL factory are encoded.