Sorry, I wanted to talk without load.
Actually we have a HA in production, And we have this behavior.
So, We made lab with two 110C FG and we can see the same behavior.
Sorry, what do you mean by 'no charge'?
If it is in lab and you are testing this, fine.
If this is just a test bed and you are planning for a production setup, and if you really want to use the HA for optimum load balancing, I would suggest virtual clustering with VDOMs.
This is possible in a-p mode where each device will be master for specific VDOMs and other device will serve as Slave unit.
The strange thing is that this environment is lab, has no charge. :(