Hello,
I have working wired AP ( xiaomi router in AP mode ) connected to openwrt which is running DHCP server. I am able to use internet as usual. Sometime, when i would like to login to AP configuration page.
I try to search for leased ip in Active DHCP list in luci, but i realsed that all of my AP ip are not appearing on the list.
I am able to search for ip using "fing" apps on my android phone though.
I would like to ask , if it is normal for openwrt for not showing AP ip address?
Either the host is configured statically, or obtained a lease before you rebooted OpenWrt.
I have reboot openwrt. But it is still missing. It is not affecting any performance, but i just find it strange that it is not shown in Opwnwrt. I can still use other ip scanner apps to look for my Access Point Ip address though. 
The wired clients can keep their old DHCP leases obtained before you rebooted OpenWrt, while the lease file is reset on reboot.
Reboot the AP and wait until it boots. Then look at the active leases in OpenWrt. It would be best to assign the IP to a static dhcp lease so you don't have to look all the time which IP it got.