DHCP users getting disconnected while accessing the internal servers

We are encountering issues with our DHCP users that are accessing the internal server. As per observation when we reserved the IP to their devices, they can connect with no issues, however if we delete the reserved IP, issue can be seen again. Please take note that we have a same model/version and configuration Fortigate which is also in our production but does not encounter this issue. We already replaced the orignal unit with our service unit, restored and upgraded to the Firmware of the original Firewall and restored the configuration but it still encountering the issue.