Traffic Sent but No Received in Forward Traffic Logs

We have a FortiGate 400F v7.0.11 running HA a-a, with 3 ISP SD-WAN. We've encountered this issue multiple times now where users cannot connect to the Internet. From what we noticed, we have around 100k 

CPU Sessions at this time. Other resources such as CPU and Memory util are normal. 

Bandwidth:

1st ISP = 600mpbs, 2nd ISP = 400mbps, 3rd ISP = 40mbps.

(Screenshot at the point where issue occured)