FAILED failover: Virtual servers were disabled on standby unit.

Re: can you share the log to search for this alert?

Change in pool member state due to monitoring is logged by syslog in /var/log/ltm. The snmp traps are sent by another process on bigip named alertd. Alertd has built-in trap events for pool members marked up or down by monitors (and many other events) which will send to the snmp trap destination defined on bigip. Alertd can also send email alerts directly from bigip. See this reference material on askf5.com

I agree that if the curl requests returned the expected results on LB1 then some internal failure stopped the health monitors from properly performing their checks.