Iphone 8/X Sends a DHCP release command

Here's one for you guys. We have detected a issue with Iphone X & 8. When moving between Wi-Fi networks it sends a DHCP release packet, this de-authenticating from the previous network. This might sound normal but it's not. The device should be able to connect back to the first network without having to connect through the captive portal, as long as it is within the DHCP lease time originally granted. Older iPhones and android exhibit this normal behavior and do not send DHCP release packets.

Any clues?

Thank you.