Client resolution in Pi-Hole's Admin Panels is based on IP addresses, not device identity.
Each of your devices might have one IPv4 and one or more IPv6 addresses -
link-local (fe80:), ULA (e.g. fd00:) and public (e.g. 2003:) prefix addresses would be common.
What you see are indeed valid hostnames for IPv6 addresses as constructed by your devices from information provided by your ISP (which seems to be Deutsche Telekom, judging by the name).
I changed my router config and now I have some entries with the IPv6 addresses and some with dip0.t-ipconnect.de.
Pi-hole should be able to show these hostnames and also to combine the requests from different IPs under one hostname.