Fortinet TAC requires below details to investigate the issue further,
Provide the below from both the HA units in 2 separate files:
#get system status
#get system performance status
#diag sys top 1 40 (Run for 30 Sec and CTRL C to stop)
#diag sys top-summary (Run for 30 Sec and CTRL C to stop) -----> only works on version 5 and need to troubleshoot memory issues
#diagnose autoupdate versions
#diagnose hardware sys shm
#diag hard sys mem
#get sys ha status
#diag sys ha showcsum
#diag hard sys slab
#diag sys session stat
#diag firewall statistic show
#diag debug crashlog read
TAC will investigate the logs and can expect the update. Ideally the split brain happens if the HA connectivity fails. Suggested to verify the physical connectivity before reach out the TAC.