Not to beat the dead IPv6 horse, but I think if you disable IPv6 on your network, this problem will no longer exist and then you should be able to assign the groups you want to assign in the group management tab.
I have multiple iPhone, iPad, iWatch, Macbooks and Mac Minis on my network, all on IPv4 only, avoiding these issues.
As noted in other replies, there is little value to have IPv6 on your LAN, and in this case it is causing problems.
Let me make one thing clear, please. I'm not here to get support. I'm here because I'm testing a beta and found something that might interest you. It's not my favorite project, I'm just hacking around. Still friends?
@jfb Yes you are completely correct, if I deactivate at the gateway Ipv6 it all works fine. It is really a wonderful piece. But if Pi-Hole is to have a future, it cannot be the solution.
@Bucking_Horn Are you still interested? What's so interesting about it, what do you want to check?
Could it be enough if I tell you that I currently have 510 configured entries. 53 of these are NICs with a statically assigned IPv4 from the Pi-Hole DHCP. 42 more are fe80 :: link-local address. 414 others all start the same as the SLAAC from Pi-Hole. And the missing one is for the IPv4 localhost?
Eureka! I've got it. It is the SLAAC Privacy Extension. The extension is made from the 64 time stamp and the MAC address to a HASH which becomes the second part of the SLAAC. The SLAAC is valid as long as it is used, after which it is replaced by a new one. And now think of intelligent battery management. Bingo! Yes, I'm pretty sure you're right.
The bad thing is that I can only fix it if I:
If I can deactivate.*
If I can't deactivate.**
By "not being able to deactivate" I mean Internet connections which depend on IPv6 or are only IPv6. Um and yes there is, I already had to install it.