Hostnames not displayed

Running a network with 4 APs based on Openwrt (19.07 or newer). APs only act as AP, so no DNS, FW, DHCP or whatever. Got used to the situation that Openwrt doesn;t show the hostname but puts an IPv6 address in that position.
Until this weekend where I was working with several RPI's, switched IP addressed many times between these devices and I suddenly observed that Openwrt started to display the real hostname.

So I had to conclude that Openwrt is able to do this correct, but ....

.... since today, the RPIs have not been touched since yesterday evening, the hostnames have disappeared and the IPv6 addresses are back again.

What do I miss ?