Hmm... I've tried adding RESOLVE_IPV6=no to /etc/pihole/pihole-FTL.conf, then I used "pihole restartdns" and (just in case) "pihole arpflush". After a few hours of waiting my Pi-hole unfortunately still used IPv6 hostnames. However, it seems like it did stop resolving some IPv6 addresses because there were quite a few of them without hostname. Retried the above steps but with rebooting afterwards but it just won't work. It stills shows the transformed addresses. That's why I commented the above.
Maybe I should just wait until MAC addresses can be used for hostnames...
edit: now the queries page (not the network table) constantly shows the correct hostname for my iPhone, but not for my Windows machine. Maybe the network table just needs to update itself for the iPhone hostname to change. I'll keep waiting.