FortiSwitch in MCLAG causes outage after reload/power cycle

Hello everyone,
 

I encountered issue where after I reload on of my core switches I lose connection to Access Switch even tho its connected redundantly to my other Core switch. 

This is diagram of the connection:

I am running 400F in HA cluster in Active-Passive mode. From both Fortigates I have Fortilink towards my Core switches. The switches are in MCLAG stack with Fortilink split interface disabled. We connected multiple access switches to the Core stack and they all link up correctly, they have been discovered by Switch Controller on 400F and they created the trunk interfaces towards the Core switches. (automatically)

When we reload CORE1 for example we lose connection to the access switch for the time the CORE is being reloaded. We did some troubleshooting and were checking STP states on CORE2 and state of the trunks during the reload. We noticed weird thing when connected to CORE2 via CLI while CORE1 was reloading → We ran some diag commands for trunks and the trunk information was missing for some of the switches. Additionally the sync between the 400F and his secondary HA unit also drops for the time and the cluster is out of sync for some reason which is weird as we are reloading one of the CORE switches and the sync should not be affected (?)

This outage also applies to the Data Plane when tested the users connected to the affected switch weren't able to ping anything. Am I missing some sort of additional configuration regarding this? I have discussed this with my colleagues and we were throwing ideas around but with no avail. 

I am little confused as the trunks and the inter connections between the switches happened automatically and the switches created their own trunks between each other. Is there a way I can run some tshoot commands to find out what is actually happening? It seems that the Access switch has some sort of connection to the FortiLink (FGT 400F) via CORE1 and when CORE1 is reloaded it does not automatically switch his link to CORE2, but I am not really sure. Any help on this matter is much appreciated. Thank you. I can provide additional details if needed but this sums up the issue so far.