Can confirm, there appears to be a bug in 386.4 which obliterates the DHCP host names list. I am able to reproduce it 100% of the time. When you open a device from the network map which does NOT have "Mac and IP Address Binding" turned on, and you enable that toggle, the DHCP host names list is wiped out. Restoring a settings backup restores the host names. DHCP reservations can be made via the DHCP server page without issue, this only occurs when the "Mac and IP Address Binding" toggle is used from that particular popup window in the network map (or AIMESH device list).
Exact STR (requires at least one device connected without an existing DHCP reservation):
1. select Network Map from router settings page
2. select Clients
3. click on the icon for any device in the list that does NOT already have a DHCP reservation
4. ensure "Mac and IP Address Binding" is not already toggled on (if it is, bug won't replicate)
5. Toggle "Mac and IP Address Binding" to ON (leave the host name and other settings alone)
6. go to LAN -> DHCP Server
7. Observe that all DHCP host names have been deleted. Adding new names works, but old ones don't come back.
I am seeing this on an RT-AX86U with an RT-AC86U AIMESH node both running 386.4, unsure if this is relevant to the bug. This issue only occurred since 386.4 and I note from the changelog, the format of the dhcp list changed here to adopt ASUS's new format.