How does iOS decide between DHCPv4 and DHCPv6

If I understood correctly, when a device connects to a network (wireless in my case), it will send both a DHCPv4 and DHCPv6 (if supported). I've setup openwrt on my router in order to view the DHCP requests (with wireshark). I have that windows clients consistently send both DHCP and DHCPv6 requests, but it looks like iPhones (iOS 14) never do. (DHCPv6 is not supported on Android, it seems) Now, my router doesn't have a DHCPv6 sever, but I assumed it doesn't matter because I'm not interested in the station actually acquiring an address, I just want to see the request. How does an iOS device decide whether to send a DHCPv6 request, and how can I force it to do so? Does the iOS device somehow know that there's no DHCPv6 server before sending a request?