Users automatically using another SBC without any settings change

Dear Experts:

During HA Testing of SBC, when one SBC_1 down then traffic shifted to another SBC_2 and after testing the settings were revert back to SBC_1 and traffic passed through it.

After some time many users automatically used SBC_2 without any settings change.

What can be the reasons?

Any manual intervention for reverting back SBC1?

No manual intervention.

In the pco 2 SBC addresses are sent if one is unavailable.

UE uses the other one.

So it is normal that untill IMS pdp broke down and setup again.

UE uses the last working SBC.