Hallo,
I have a similar problem with german umlauts. I ingest with PM applying IPTC stationery pad to the CR2-files, import them into PS Lightroom and add some IPTC-Data (caption field) among other things and then I export JPGs.
In PM all JPG's umlauts seem to be o.k.. But in some WEB based databanks the umlauts mutate into hieroglyphs.
What IPTC/XMP preference settings are advisable and should help "tools -> update IPTC/XMP" ?
-Regards