I'm getting the same issue.
We have two Fortigates 1801F in HA.
In the past, the 2 incident only occurred in the same unit. But now, it has ocurred on the other unit.
When a unit is the Primary HA, without doing any changes, the communications drops and we have to change to the other node as Primary to recover the service. The only option is reboot the Primary unit to change the HA.
In January was reproduced with the 7.0.9 version. Now, we upgraded to v7.0.10, and with the new version, the incident has returned.
I don't see any error logs.
It seems that the unit doesn't accept new connections, and only works the old connections. For exemple, If I'm connected via GUI when the incident occurs, I can navigate through the GUI, but I can't do ping or connect via SSH.
This incident not occurs always.