This might be useful to others, so I'm sharing it as I don't know how to make a change like this acceptable upstream. The current version of the patch is not high quality, just a quick and dirty way to make it compatible.
OpenWrt's DHCPv6 client (odhcp6c) doesn't receive any response to 'solicit' messages from Huawei DHCPv6 servers at least with a friend's ISP (Telekom Srbija's Supernova sub-brand in bridge mode).
dhcpcd on the other hand works fine and gets a response for both IA_NA and IA_PD.
Reordering headers in the packets makes the server respond to odhcp6c's requests and gets the assigned NA and PD.