Priority-group does not seem to work

Hello, I use 2 Link Controllers with BIGIP V12 HF2

I am trying to use Priority Group feature on a Pool that represents the 2 Default ISP routers available to reach Internet;

For some specific cases (such as IPSEC VPN outgoing requests), I want BigIP to use only the 1st ISP router if it is available, and start to use the 2nd ISP router if the Internet access thru the 1st one is detected as unreachable ;

So, I created the following kind of Pool :

ltm pool /Common/Default-Gateway-via-ISP1+Backup-via-ISP2 { 
   members { 
       /Common/RTR-ISP1:0 { 
           address 111.1.1.1 
           priority-group 100 
       } 
       /Common/RTR-ISP2:0 { 
           address 222.2.2.2
           priority-group 1 
       } 
   } 
   min-active-members 1 
   monitor /Common/Ping-DNS1-Google and /Common/Ping-DNS2-Google
}   

When I test the Virtual Server that uses this Pool, I can see, via TCPDUMP, that outgoing flows are always load-balanced on both ISP links instead of using ISP1 link only; so, it does not seems to work ...

To be complete, the VS is defined with dest 0.0.0.0/0, with a specific Source IP, and with a SNAT Pool of 2 public IPs associated to this Source IP

This is the 1st time I use Priority Group feature

Any idea ?