iOS Devices Not Getting IP via DHCP Relay on FortiGate 30E

Description:

Hello everyone,

I’m facing a strange issue with our FortiGate 30E (firmware version 6.2.16). One of our internal staff networks uses a DHCP relay configuration. The relay forwards DHCP requests to a central DHCP server, from which devices such as computers and phones receive their IP addresses.
Today we encountered a problem where some iOS devices (iPhone and iPad) couldn’t connect to the network via DHCP.
Here’s what we observed:

Not working:
iOS 18.7.1
iOS 18.6.2

Working fine:
iOS 18.3.2
iOS 18.5
iOS 26.01

It seems that only newer iOS versions are failing to obtain an IP address through DHCP relay.

Below is the configuration of the DHCP relay on the FortiGate interface:

set dhcp-relay-ip "10.148.102.102"
set dhcp-relay-type regular
set dhcp-relay-agent-option enable
set mtu-override disable
set wccp disable
set drop-overlapped-fragment disable
set drop-fragment disable
set interface "lan"
set vlanid 79

Has anyone experienced a similar issue where newer iOS versions fail to obtain an IP address through DHCP relay on FortiGate?
Could this be related to DHCP relay agent options, MTU size, or recent iOS DHCP client behavior changes?

Any insights or experiences would be greatly appreciated!
FortiGate