Hostnames are changing to Hardwarenames

I have been using pihole for a long time and everything works perfectly. Since the last update, however, I have noticed something regarding the hostnames.

The master in my network is a Fritzbox 6490, which takes care of DHCP. When a new device is added, it identifies itself with its hardware name, which I then change to a more unique name.

Until the last update, the device list in Pihole used exactly the device names I entered myself. When I restart Pihole now, this is still the case. After a few hours, however, the list only shows the hardware names. So my TVs are no longer ‘tv-livingroom’ and ‘tv-guestroom’ but only “Android” and ‘Android2’.

What is causing this?
How can I change this?