Thanks!
I tried dig with other devices but still having same result, my router can show the name in it's UI just fine, just like you said. Still, I'm unsure if using pihole as DHCP server can cause any performance issue.
I don't have 2 DHCP in my network but before I was using the main modem as DHCP (with pihole it works fine!) then I bought the Tenda router and move all cable to it and use it as DHCP in different subnet then issue start to happen.