it doesn't work since the ARP-NAT that is done by Openwrt to allow traffic in client mode is designed for IPv4 only.
So, it really is a link level problem, which affects everything. But a hacky workaround is able to hide the link level problem in case of IPv4.
So it is not the problem that is IPv6 specific, it is the workaround that is IPv4 specific.
WDS however allows it. Why not use WDS?!
Yeah, from the linked description, WDS sounds more like a solution and less like a hack. But the description also mentions that WDS is non-standard and doesn't work well with equipment from different vendors.
Also, something doesn't sound right about WDS either. Why does the MAC addresses of the sending and receiving AP have to be in the frame in the first place? It is a wireless link, the frame is going to be received by the radio of all APs in range on the frequence regardless. And instead of looking at the MAC of the destination AP it could just look on the MAC of the destination host. If packets for that destination MAC would be sent over one of the switch ports on this AP, then the packet is for this AP. If the packets for that destination MAC would be sent over the wireless interface, then it is not for this AP. That could eliminate the destination AP MAC address, and since the only purpose of the sending AP MAC address is to know what to put in the destination AP MAC address for return traffic, then both of them could be eliminated, bringing it down to just two MAC addresses, namely the two shown in red in the figures.