After trying to unplug the online feb module, it will not automatically switch to another feb module.
This is when PSM is in 1+1 redundancy state
Number of FEBs required for operation
ACX7509-BASE systems: one FEB
ACX7509-PREMIUM systems: two FEBs
If you want to run 2 RCB + 2 FEB, which is Premium, you need 2+2 PSM redundancy as per document.
https://www.juniper.net/documentation/us/en/hardware/acx7509/topics/concept/acx7509-feb.html
When you have 2+2 PSM, the FEB will switchover fine to standby.