I have seen this behavior occasionally. In most cases, a browser refresh clears it. You might also try clearing the browser cache and then reloading the web admin page.
Not directly related to your problem, but a common question:
If you don’t like the client hostnames that are shown (these are the names Pi-Hole gets during the DHCP handshake), you can enter the clients in your /etc/hosts file on the Pi and put a client name there. This is helpful if the client name that Pi-Hole has is not descriptive.
An example might be a printer - the printer tells Pi-Hole that is device “BrotherHL-5370W-wireless”, but you want it to show as “office-printer”.
Edit the /etc/hosts file (sudo nano etc/hosts) and add a new line with the IP for that device and the name you want:
Save and exit.