Well, I am experiencing something extremely similar.
Using nopC 4.40.4
Have two currencies. USD and CAD. USD is the primary store currency. Structured data always comes back CAD. If I disable the ECB exchange provider then structured data returns correct. My setting of autodetect language is not enabled. My google indexed product pages show CAD for all my products even when browsing from the US. Using another SEO tool such as Screaming Frog's SEO spider produces the same results. In my case, do not believe this has to do with anything geo related. Using my VPN, can query the structured data from anywhere in the world and it always comes back CAD.
Using no source version so cannot try code above. :( Don't know if this coming from a plugin or the from the latest minor release. Been disabling one at a time and testing. Maybe comparing installed plugins? I use FoxNet and NopTemplate plugins. Test with a fresh demo install I guess.
