MySQL active connection never bleed off to other pool member

I am running galera MySQL behind F5 with

performance Layer 4

type and i have setup 3 mysql node in pool member with

Priority

so only 1 mysql node will be used and other two will be standby.

So everything was good but i found today when i shutdown Primary node which was active and i found my application break and when i have checked logs found:

(2006, "MySQL server has gone away (error(104, 'Connection reset by peer'))")

So solution was restart application, look like active member mysql connection not bleeding off to other pool member, what is wrong with my setup?