Restart TMM frequency

Please check the vlan failsafe configurations vlan failsafe timeout value is configured as low, change that value

Also the network settings have to configure on both boxes individually as they don't replicate so change on both box settings 

Is it reboot setting than you need to disable it on the relevant vlans 

https://my.f5.com/manage/s/article/K13297

check the /var/log/ltm

also upload the qkview to ihealth.f5.com.
it may provide some analysis

Hi SanYang,

Have you done the license reactivation on the F5 before starting reboot to the new OS partition, if not done, please check and boot back to old partition from console port and fix the license reactivation.or if severity please open a TAC case for live tshoot.

Also  it is highly recommended to Force Offline the standby unit while you upgrade it so that it will not declate itself as Active if HA issues. When you are ready to upgrade the primary, before Force Offline the primary you have to  Release Offline on the standby and do a failover from primary to make it standby and now once Primary is now standby & the upgraded Standby will be Active, now you can force offline the primary which is standby now, although in F5 Primary /Secondary are not used in F5 documentation, they use Active/Standby, but for our ease we can mark who is Primary before we start the OS upgrade activity and who is Seconday and based on that we can mark Active /Standby and switch the roled after failover. Also remember the reason stated is that HA Active/Standby  will not work properly on two different versions. There you will see HA state as Disconnected.

Please write back for more help.

HTH

🙏