DHCPD hostnames have stopped working

[jonm]

 I've noticed that DHCP clients aren't showing hostnames any more, for new clients. Old ones are working fine. So for example my iphone shows as Jons-iphone-XS but a visitor to the house who has connected to the wifi just has a blank in the hostname column.

I've not changed anything but I have been regularly updating and am currently running 22.1.9_1.

Where should I be looking to try and figure out what's causing this, please?