Fortigate behavior changes without apparent reason.

Hello everyone. In our infrastructure, we have a Fortigate 100f. Situation: we are replacing our main switches. I have attached an image for you to see the changes. Basically, we want to remove core-1 and core-2 switches and will keep core-3 and core-4. Currently, everything has been migrated except for VLAN 1, where the firewall and the network communications of the switches are located. The only change I really see here is in MAC addresses since we are configuring the VRRP group in the new cores. That said, when we move VLAN 1 to cores 3 and 4 to finally shut down cores 1 and 2, everything works perfectly except for the services we have in the DMZ of our Fortigate. The question I have is, what could be changing if currently everything is working including the DMZ, and when we make the switch, everything works except the Fortigate's DMZ. Basically, the behavior of the Fortigate is that it sends the traffic, which it is currently correctly sending to the LAN, to another interface or towards the internet. I would appreciate some help on this, and if you need more details, please do not hesitate to ask.