It's not particularly unusual for domestic segments to book into a higher booking class than international segments on a through fare - all the major Chinese carriers (HU, CA, MU, CZ) do it, and it's common on other countries as well (e.g. AI does it heavily in India). The purpose is to ensure that international flights don't end up flying empty because the shorter domestic connecting flights sold out.

US carriers accomplish the same thing through married-segment inventory controls.