About F5 Cluster Config Sync and save changes

Hi according to the documentation you can change default behavior (https://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/bigip-device-service-clustering-admin-11-5-0/7.htmlunique_589421575) :

Alternatively, you can change the default behavior so that the system automatically saves configuration changes on >target devices after an automatic config sync. You make this change by logging into one of the devices in the device >group and, at the tmsh prompt, typing modify cm device-group name save-on-auto-sync true.

Warning: Enabling the save-on-auto-sync option can unexpectedly impact system performance when the BIG-IP system >automatically saves a large configuration change to each device.

Anyway if one device goes off, if you log into the other and you makes a change on it this will will save the config in that device again. Think about the situation where you have both devices off is really really small, that never happened to me.