HA Issue on #FortiGate 600E

We're currently facing an issue with our #FortiGate 600E firewalls configured in HA (High Availability) mode. The setup was working fine before, but recently we've noticed consistent failures in the HA behavior under certain conditions. For example

When we perform a basic action like ending a user session,

When a user with read-only access interacts with the firewall,

Or when we reboot the secondary firewall  it unexpectedly becomes the primary, and the HA fails every time.

These issues didn’t occur previously, and we haven't made any major configuration changes to the HA setup.

Could you help us understand what's causing this and suggest a solution to restore stable HA functionality?

Thank you!